Summary:
This PR adds a help class block cache tracer to read/write block cache accesses. It uses the trace reader/writer to perform this task.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5410

Differential Revision: D15612843

Pulled By: HaoyuHuang

fbshipit-source-id: f30fd1e1524355ca87db5d533a5c086728b141ea