Summary:
Range Locking - an implementation based on the locktree library

- Add a RangeTreeLockManager and RangeTreeLockTracker which implement
  range locking using the locktree library.
- Point locks are handled as locks on single-point ranges.
- Add a unit test: range_locking_test

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7506

Reviewed By: akankshamahajan15

Differential Revision: D25320703

Pulled By: cheng-chang

fbshipit-source-id: f86347384b42ba2b0257d67eca0f45f806b69da7