Add support for the 'TicketBatch' amendment:

Support for 'out-of-sequence' transaction execution was introduced
in commit 7724cca384.

The changes in that commit were gated under a feature but there was
no corresponding amendment introduced that would allow the network
to vote on this amendment.

This commit introduces 'TicketBatch' amendment as the amendment
that is associated with the tickets feature. If the amendment is
enabled, it will activate support for tickets.

This commit also removes several workarounds that are no longer
needed in unit tests.

src/ripple/protocol/impl/Feature.cpp

@@ -132,7 +132,7 @@ detail::supportedAmendments()
         "HardenedValidations",
         "fixAmendmentMajorityCalc",
         //"NegativeUNL",      // Commented out to prevent automatic enablement
-        //"TicketBatch",      // Commented out to prevent automatic enablement
+        "TicketBatch",
     };
     return supported;
 }