[RocksDB] Optimize locking for Get

Summary:
Instead of locking and saving a DB state, we can cache a DB state and update it only when it changes. This change reduces lock contention and speeds up read operations on the DB.

Performance improvements are substantial, although there is some cost in no-read workloads. I ran the regression tests on my devserver and here are the numbers:

  overwrite                    56345  ->   63001
  fillseq                      193730 ->  185296
  readrandom                   771301 -> 1219803 (58% improvement!)
  readrandom_smallblockcache   677609 ->  862850
  readrandom_memtable_sst      710440 -> 1109223
  readrandom_fillunique_random 221589 ->  247869
  memtablefillrandom           105286 ->   92643
  memtablereadrandom           763033 -> 1288862

Test Plan:
make asan_check
I am also running db_stress

Reviewers: dhruba, haobo, sdong, kailiu

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14679