…Avoid looking up symbols.

Key part: Majority of query latency comes from symbols.Lookup.

Also cleaned up the code.

## Benchmarks

Baseline:
```
/tmp/___BenchmarkTelemeterRealData_Series_in_github_com_thanos_io_thanos_pkg_store -test.v -test.bench ^\QBenchmarkTelemeterRealData_Series\E$ -test.run ^$ -test.benchtime=1m -test.benchmem
goos: linux
goarch: amd64
pkg: github.com/thanos-io/thanos/pkg/store
BenchmarkTelemeterRealData_Series
Built index header; Starting
BenchmarkTelemeterRealData_Series/alerts2w/01DN3SK96XDAEKRB1AN30AAW6E
BenchmarkTelemeterRealData_Series/alerts2w/01DN3SK96XDAEKRB1AN30AAW6E-12         	      43	1866550478 ns/op	395648807 B/op	 4348808 allocs/op
BenchmarkTelemeterRealData_Series/alerts15s/01DN3SK96XDAEKRB1AN30AAW6E
BenchmarkTelemeterRealData_Series/alerts15s/01DN3SK96XDAEKRB1AN30AAW6E-12        	      50	1417560470 ns/op	160595177 B/op	 3017177 allocs/op
BenchmarkTelemeterRealData_Series/subssyncs2w/01DN3SK96XDAEKRB1AN30AAW6E
BenchmarkTelemeterRealData_Series/subssyncs2w/01DN3SK96XDAEKRB1AN30AAW6E-12      	     392	 168130862 ns/op	49921099 B/op	  493076 allocs/op
BenchmarkTelemeterRealData_Series/subs2w/01DN3SK96XDAEKRB1AN30AAW6E
BenchmarkTelemeterRealData_Series/subs2w/01DN3SK96XDAEKRB1AN30AAW6E-12           	      14	5295236741 ns/op	1083815895 B/op	11798226 allocs/op
BenchmarkTelemeterRealData_Series/subs15s/01DN3SK96XDAEKRB1AN30AAW6E
BenchmarkTelemeterRealData_Series/subs15s/01DN3SK96XDAEKRB1AN30AAW6E-12          	      18	4149971387 ns/op	436568360 B/op	 8577264 allocs/op
PASS

Process finished with exit code 0
```

New
```
benchstat -delta-test=none _dev/bench/store_symb_a.txt _dev/bench/store_symb_b.txt
name                                                                old time/op    new time/op    delta
TelemeterRealData_Series/alerts2w/01DN3SK96XDAEKRB1AN30AAW6E-12        1.87s ± 0%     1.66s ± 0%  -10.85%
TelemeterRealData_Series/alerts15s/01DN3SK96XDAEKRB1AN30AAW6E-12       1.42s ± 0%     0.25s ± 0%  -82.17%
TelemeterRealData_Series/subssyncs2w/01DN3SK96XDAEKRB1AN30AAW6E-12     168ms ± 0%     157ms ± 0%   -6.72%
TelemeterRealData_Series/subs2w/01DN3SK96XDAEKRB1AN30AAW6E-12          5.30s ± 0%     4.53s ± 0%  -14.53%
TelemeterRealData_Series/subs15s/01DN3SK96XDAEKRB1AN30AAW6E-12         4.15s ± 0%     0.68s ± 0%  -83.67%

name                                                                old alloc/op   new alloc/op   delta
TelemeterRealData_Series/alerts2w/01DN3SK96XDAEKRB1AN30AAW6E-12        396MB ± 0%     396MB ± 0%   -0.00%
TelemeterRealData_Series/alerts15s/01DN3SK96XDAEKRB1AN30AAW6E-12       161MB ± 0%      95MB ± 0%  -40.95%
TelemeterRealData_Series/subssyncs2w/01DN3SK96XDAEKRB1AN30AAW6E-12    49.9MB ± 0%    49.9MB ± 0%   +0.00%
TelemeterRealData_Series/subs2w/01DN3SK96XDAEKRB1AN30AAW6E-12         1.08GB ± 0%    1.08GB ± 0%   -0.00%
TelemeterRealData_Series/subs15s/01DN3SK96XDAEKRB1AN30AAW6E-12         437MB ± 0%     228MB ± 0%  -47.77%

name                                                                old allocs/op  new allocs/op  delta
TelemeterRealData_Series/alerts2w/01DN3SK96XDAEKRB1AN30AAW6E-12        4.35M ± 0%     4.35M ± 0%   -0.00%
TelemeterRealData_Series/alerts15s/01DN3SK96XDAEKRB1AN30AAW6E-12       3.02M ± 0%     0.05M ± 0%  -98.36%
TelemeterRealData_Series/subssyncs2w/01DN3SK96XDAEKRB1AN30AAW6E-12      493k ± 0%      493k ± 0%   +0.00%
TelemeterRealData_Series/subs2w/01DN3SK96XDAEKRB1AN30AAW6E-12          11.8M ± 0%     11.8M ± 0%   -0.00%
TelemeterRealData_Series/subs15s/01DN3SK96XDAEKRB1AN30AAW6E-12         8.58M ± 0%     0.02M ± 0%  -99.78%
```
Cmp:

```
benchstat -delta-test=none _dev/bench/store_symb_a.txt _dev/bench/store_symb_b.txt
name                                                                old time/op    new time/op    delta
TelemeterRealData_Series/alerts2w/01DN3SK96XDAEKRB1AN30AAW6E-12        1.87s ± 0%     1.66s ± 0%  -10.85%
TelemeterRealData_Series/alerts15s/01DN3SK96XDAEKRB1AN30AAW6E-12       1.42s ± 0%     0.25s ± 0%  -82.17%
TelemeterRealData_Series/subssyncs2w/01DN3SK96XDAEKRB1AN30AAW6E-12     168ms ± 0%     157ms ± 0%   -6.72%
TelemeterRealData_Series/subs2w/01DN3SK96XDAEKRB1AN30AAW6E-12          5.30s ± 0%     4.53s ± 0%  -14.53%
TelemeterRealData_Series/subs15s/01DN3SK96XDAEKRB1AN30AAW6E-12         4.15s ± 0%     0.68s ± 0%  -83.67%

name                                                                old alloc/op   new alloc/op   delta
TelemeterRealData_Series/alerts2w/01DN3SK96XDAEKRB1AN30AAW6E-12        396MB ± 0%     396MB ± 0%   -0.00%
TelemeterRealData_Series/alerts15s/01DN3SK96XDAEKRB1AN30AAW6E-12       161MB ± 0%      95MB ± 0%  -40.95%
TelemeterRealData_Series/subssyncs2w/01DN3SK96XDAEKRB1AN30AAW6E-12    49.9MB ± 0%    49.9MB ± 0%   +0.00%
TelemeterRealData_Series/subs2w/01DN3SK96XDAEKRB1AN30AAW6E-12         1.08GB ± 0%    1.08GB ± 0%   -0.00%
TelemeterRealData_Series/subs15s/01DN3SK96XDAEKRB1AN30AAW6E-12         437MB ± 0%     228MB ± 0%  -47.77%

name                                                                old allocs/op  new allocs/op  delta
TelemeterRealData_Series/alerts2w/01DN3SK96XDAEKRB1AN30AAW6E-12        4.35M ± 0%     4.35M ± 0%   -0.00%
TelemeterRealData_Series/alerts15s/01DN3SK96XDAEKRB1AN30AAW6E-12       3.02M ± 0%     0.05M ± 0%  -98.36%
TelemeterRealData_Series/subssyncs2w/01DN3SK96XDAEKRB1AN30AAW6E-12      493k ± 0%      493k ± 0%   +0.00%
TelemeterRealData_Series/subs2w/01DN3SK96XDAEKRB1AN30AAW6E-12          11.8M ± 0%     11.8M ± 0%   -0.00%
TelemeterRealData_Series/subs15s/01DN3SK96XDAEKRB1AN30AAW6E-12         8.58M ± 0%     0.02M ± 0%  -99.78%
```

Code: https://gist.github.com/bwplotka/cbcbbcd1802181b7785da11dcc0f5cfd


Signed-off-by: Bartlomiej Plotka <bwplotka@gmail.com>