[rocksdb] Regression tests

Summary:
* Fixed regression test params by @dhruba's suggestion
* Added p50, p75 and p99 to regression metrics

Test Plan: build_tools/build_regression_test.sh

Reviewers: dhruba, emayanke

Reviewed By: dhruba

CC: leveldb, dhruba, reconnect.grayhat

Differential Revision: https://reviews.facebook.net/D14355