Dataflow Locks

[KinanBab]

• [ ] Dataflow parallelism may be superflous
• [ ] Improve locks: a flow-wide lock for writers only, a reader-writer lock for matview
• [ ] Fork Rocksdb and provide a commit that keeps locks unreleased.
• [ ] Use fork to update dataflow while holding DB locks, then release everything together.

Forking Rocksdb:

1. Remove the Clear() call from Commit() function in rocksdb/utilities/pessimistic_transaction.cc. Consider using a flag to disable/enable calling clear.
2. Expose Clear() so that we can call it manually from Pelton after committing. Consider using destructor or rollback.
3. Test via logging / monitoring of locks.
4. Open issue/PR?