Apply transaction batches in periodic intervals (#4504)

Add new transaction submission API field, "sync", which
determines behavior of the server while submitting transactions:
1) sync (default): Process transactions in a batch immediately,
   and return only once the transaction has been processed.
2) async: Put transaction into the batch for the next processing
   interval and return immediately.
3) wait: Put transaction into the batch for the next processing
   interval and return only after it is processed.

src/ripple/app/main/Application.cpp

@@ -1527,6 +1527,7 @@ ApplicationImp::start(bool withTimers)
     {
         setSweepTimer();
         setEntropyTimer();
+        m_networkOPs->setBatchApplyTimer();
     }
 
     m_io_latency_sampler.start();