Switch from keydb to dragonfly in CI - #2907

There are a few edge cases where Dragonfly has slightly different
functionality or returns a slightly different RESP shape. I'll open an
issue with them to see whether they want to be more compatible.

In the meantime they're pretty rare edge cases so we can handle them
here so we can run the vast majority of tests against Dragonfly now.

We've got a generic multibulk handler that didn't handle elements that
were non-strings. This seems like a bug in general but also fixes it so
that our reply handler works for `TIME` in both `Redis` and `Dragonfly`

This fixes a test failure when running against Dragonfly but is also an
improvement over just using PHPREDIS_CTX_PTR variants to figure out what
the reply handler should do.

Previously the test just iterated 1000 times getting a randomkey and
then ensuring that the key existed. This will pretty much always work if
you run the whole test suite, but running it in isolation on an empty
dtabase would fail.

```php
// Returns false
$key = $redis->randomkey();
$this->assertKeyExists($key); // fails
```