Rolling back a transaction after commit timeout should release locks in two phases

This is related to ISPN-4137.

When a commit command times out, we send a rollback command to release locks. The problem is that the node that timed out might still execute commit command, before the rollback command reaching the backup but after it released the locks on the primary owner.

The solution is to not release any locks during the rollback command's execution, but send an asynchronous TxCompletionNotification afterwards - just like for commit commands.