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/test/rpc/RobustTransaction_test.cpp

@@ -17,6 +17,7 @@
 */
 //==============================================================================
 
+#include <ripple/app/misc/NetworkOPs.h>
 #include <ripple/beast/unit_test.h>
 #include <ripple/core/JobQueue.h>
 #include <ripple/protocol/jss.h>
@@ -88,7 +89,8 @@ public:
             }
             BEAST_EXPECT(jv[jss::result][jss::engine_result] == "tefPAST_SEQ");
 
-            // Submit future sequence transaction
+            // Submit future sequence transaction -- this transaction should be
+            // held until the sequence gap is closed.
             payment[jss::tx_json][sfSequence.fieldName] = env.seq("alice") + 1;
             jv = wsc->invoke("submit", payment);
             if (wsc->version() == 2)
@@ -114,6 +116,8 @@ public:
             }
             BEAST_EXPECT(jv[jss::result][jss::engine_result] == "tesSUCCESS");
 
+            // Apply held transactions.
+            env.app().getOPs().transactionBatch(true);
             // Wait for the jobqueue to process everything
             env.app().getJobQueue().rendezvous();