Expose number of entries in mem tables to users

Summary: In this patch, two new DB properties are defined: rocksdb.num-immutable-mem-table and rocksdb.num-entries-imm-mem-tables, from where number of entries in mem tables can be exposed to users

Test Plan:
Cover the codes in db_test
make all check

Reviewers: haobo, ljin, igor

Reviewed By: igor

CC: nkg-, igor, yhchiang, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D18207