Fixes stop() process in TransactionCommittingResponseUnpacker

TCRU uses the TransactionIdStore to update committing and applying
 tx ids, as expected by the tx commit protocol. However, if a stop()
 was issued, the txIdStore field would be set to null and if this
 happened during a tx apply where the txIdStore.transactionCommitted()
 call had already gone through, the subsequent transactionClosed() event
 would fail with a NPE. This would leave the txIdStore waiting for a
 transaction to apply, which would prohibit the database from shutting
 down.
 This patch removes the setting to null on stop approach and uses
 a boolean to mark the stopped state in TCRU, not allowing any
 more transactions to go through and allowing the shutdown to finish.
Merge pull request #3186 from digitalstain/2.2-fix-response-unpacker-stop

[2.2] Fixes stop() process in TransactionCommittingResponseUnpacker