Implement enhanced Ticket support:

Tickets are a mechanism to allow for the "out-of-order" execution of
transactions on the XRP Ledger.

This commit, if merged, reworks the existing support for tickets and
introduces support for 'ticket batching', completing the feature set
needed for tickets.

The code is gated under the newly-introduced `TicketBatch` amendment
and the `Tickets` amendment, which is not presently active on the
network, is being removed.

The specification for this change can be found at:
https://github.com/xrp-community/standards-drafts/issues/16

src/ripple/protocol/impl/STTx.cpp

@@ -160,6 +160,22 @@ STTx::getSignature() const
     }
 }
 
+SeqProxy
+STTx::getSeqProxy() const
+{
+    std::uint32_t const seq{getFieldU32(sfSequence)};
+    if (seq != 0)
+        return SeqProxy::sequence(seq);
+
+    boost::optional<std::uint32_t> const ticketSeq{operator[](
+        ~sfTicketSequence)};
+    if (!ticketSeq)
+        // No TicketSequence specified.  Return the Sequence, whatever it is.
+        return SeqProxy::sequence(seq);
+
+    return SeqProxy{SeqProxy::ticket, *ticketSeq};
+}
+
 void
 STTx::sign(PublicKey const& publicKey, SecretKey const& secretKey)
 {
@@ -250,8 +266,8 @@ STTx::getMetaSQL(
 
     return str(
         boost::format(bfTrans) % to_string(getTransactionID()) %
-        format->getName() % toBase58(getAccountID(sfAccount)) % getSequence() %
-        inLedger % status % rTxn % escapedMetaData);
+        format->getName() % toBase58(getAccountID(sfAccount)) %
+        getFieldU32(sfSequence) % inLedger % status % rTxn % escapedMetaData);
 }
 
 std::pair<bool, std::string>