Add a function to return current perf level

Summary: Add a function to return the perf level. It is to allow a wrapper of DB to increase the perf level and restore the original perf level after finishing the function call.

Test Plan: Add a verification in db_test

Reviewers: yhchiang, igor, ljin

Reviewed By: ljin

Subscribers: xjin, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D19551