Summary:
Avoid locking the DB mutex in order to reference SuperVersions. Instead, we get the thread local cached SuperVersion for each column family in the list. It depends on finding a sequence number that overlaps with all the open memtables. We start with the latest published sequence number, and if any of the memtables is sealed before we can get all the SuperVersions, the process is repeated. After a few times, give up and lock the DB mutex.

Tests:
1. Unit tests
2. make check
3. db_bench -

TEST_TMPDIR=/dev/shm ./db_bench -use_existing_db=true -benchmarks=readrandom -write_buffer_size=4194304 -target_file_size_base=4194304 -max_bytes_for_level_base=16777216 -num=5000000 -reads=1000000 -threads=32 -compression_type=none -cache_size=1048576000 -batch_size=1 -bloom_bits=1
readrandom   :       0.167 micros/op 5983920 ops/sec;  426.2 MB/s (1000000 of 1000000 found)

Multireadrandom with batch size 1:
multireadrandom :       0.176 micros/op 5684033 ops/sec; (1000000 of 1000000 found)
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4754

Differential Revision: D13363550

Pulled By: anand1976

fbshipit-source-id: 6243e8de7dbd9c8bb490a8eca385da0c855b1dd4