Summary:
We're going to support more locking protocols such as range lock in transaction.

However, in current design, `TransactionBase` has a member `tracked_keys` which assumes that point lock (lock a single key) is used, and is used in snapshot checking (isolation protocol). When using range lock, we may use read committed instead of snapshot checking as the isolation protocol.

The most significant usage scenarios of `tracked_keys` are:
1. pessimistic transaction uses it to track the locked keys, and unlock these keys when commit or rollback.
2. optimistic transaction does not lock keys upfront, it only tracks the lock intentions in tracked_keys, and do write conflict checking when commit.
3. each `SavePoint` tracks the keys that are locked since the `SavePoint`, `RollbackToSavePoint` or `PopSavePoint` relies on both the tracked keys in `SavePoint`s and `tracked_keys`.

Based on these scenarios, if we can abstract out a `LockTracker` interface to hold a set of tracked locks (can be keys or key ranges), and have methods that can be composed together to implement the scenarios, then `tracked_keys` can be an internal data structure of one implementation of `LockTracker`. See `utilities/transactions/lock/lock_tracker.h` for the detailed interface design, and `utilities/transactions/lock/point_lock_tracker.cc` for the implementation.

In the future, a `RangeLockTracker` can be implemented to track range locks without affecting other components.

After this PR, a clean interface for lock manager should be possible, and then ideally, we can have pluggable locking protocols.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7013

Test Plan: Run `transaction_test` and `optimistic_transaction_test`.

Reviewed By: ajkr

Differential Revision: D22163706

Pulled By: cheng-chang

fbshipit-source-id: f2860577b5334e31dd2994f5bc6d7c40d502b1b4