2-level transaction multi-signatures (RIPD-182):

This commit provides support for 2-level multi-signing of
transactions.  The ability is usually compiled out, since other
aspects of multi-signing are not yet complete.

Here are the missing parts:

 o Full support for Tickets in transactions.
 o Variable fees based on the number of signers,
 o Multiple SignerLists with access control flags on accounts,
 o Enable / disable operations based on access control flags,
 o Enable / disable all of multi-signing based on an amendment,
 o Integration tests, and
 o Documentation.

src/ripple/rpc/tests/JSONRPC.test.cpp

@@ -963,7 +963,8 @@ public:
         {
             TestStuff {transactionSign,              "sign",               0},
             TestStuff {transactionSubmit,            "submit",             1},
-            TestStuff {transactionSignFor,           "sign_for",           2}
+            TestStuff {transactionSignFor,           "sign_for",           2},
+            TestStuff {transactionSubmitMultiSigned, "submit_multisigned", 3}
         };
 
         for (auto testFunc : testFuncs)