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/protocol/impl/TER.cpp

@@ -189,6 +189,7 @@ transResults()
         MAKE_ERROR(terQUEUED,                 "Held until escalated fee drops."),
         MAKE_ERROR(terPRE_TICKET,             "Ticket is not yet in ledger."),
         MAKE_ERROR(terNO_AMM,                 "AMM doesn't exist for the asset pair."),
+        MAKE_ERROR(terSUBMITTED,              "Transaction has been submitted."),
 
         MAKE_ERROR(tesSUCCESS,                "The transaction was applied. Only final in a validated ledger."),
     };