Memory leak with MemoryCache

[cijothomas]

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.

[cijothomas]

The issue was with DependencyTableStore creating new MemoryCache as a side effect during every Instance property access. This problem existed for months now, but was aggravated with 516, as it called Instance property for every callback.