Summary:
add SIM_BLOCK_CACHE_HIT and SIM_BLOCK_CACHE_MISS tickers.
maybe can be combined with Histograms like DB_GET to evaluate the current setting of the size of block cache.

Test Plan: make all check

Reviewers: sdong, andrewkr, IslamAbdelRahman, yiwu

Reviewed By: yiwu

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D61803