Two messages from Antidote's interface to the transaction coordinator to commit a transaction

In the current state, when one wants to commit an interactive transaction, a client will issue a "commit_transaction" command, that will execute the 2PC commit protocol as follows:

1. the gen_server running the interface sends a prepare message to the coordinator
2. coord sends prepare message to the vnodes that performed updates
3. vnodes respond to the coordinator
4. coordinator responds to the gen server that runs the interface
5. the gen server sends a commit message to the coord
6. coord sends commit message to the vnodes that performed updates
7. vnodes respond to the coordinator
8. the coordinator responds to the gen_server running the interface

https://github.com/SyncFree/antidote/blob/master/src/antidote.erl#L98
https://github.com/SyncFree/antidote/blob/master/src/antidote.erl#L446

I propose removing steps 5 and 6.
Furthermore, I propose replying to the client after 6, after logging the decision to commit and the commit time at the txn coordinator for fault-tolerance.

This will reduce the latency of the transaction.