MyRocks: Range Locking: shared point lock support

Range locking should support shared locks. Supporting them in general case is hard, so the goal is to just support shared point locks.

Multiple transactions should be able to run

select ... from table where key= ... lock in share mode

Tree:
MyRocks: https://github.com/spetrunia/mysql-5.6/tree/range-locking-fb-mysql-5.6.35-shared-locks
RocksDB part: https://github.com/spetrunia/rocksdb/tree/spetrunia-range-locking-sharedlocks