Memory leak with MemoryCache #554

After PR #516 , there are multiple readers/writers to the MemoryCache - EventSource processing and DiagnosticListener processing. Its not fully understood, but there seems to be a huge memory leak and dumps show MemoryCache is the cause. The items count in cache never goes above the expected value, but their memory usage keeps increasing forever.

Replacing mem cache with own implementation is fixing this problem. Opening this GH issue to understand the issue better or to see if we can live with custom implementation.