Summary:
Current implementation of perf context is level agnostic. Making it hard to do performance evaluation for the LSM tree. This PR adds `PerfContextByLevel` to decompose the counters by level.
This will be helpful when analyzing point and range query performance as well as tuning bloom filter
Also replaced __thread with thread_local keyword for perf_context
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4226

Differential Revision: D10369509

Pulled By: miasantreble

fbshipit-source-id: f1ced4e0de5fcebdb7f9cff36164516bc6382d82