Modularize app/tx:

* Move LedgerView declaration
* Move CrossType
* Move Clock declaration
* Move Quality to protocol/
* Move Amounts to protocol/
* Move book to tx/impl
* Remove 0.27 legacy support
* Remove unused AmountType
* Remove core namespace
* Use STAmount

src/ripple/protocol/Protocol.h

@@ -46,8 +46,18 @@ struct Protocol
     static int const txMaxSizeBytes = 1024 * 1024; // 1048576
 };
 
-/** A ledger index.
+/** A clock representing network time.
+    This measures seconds since the Ripple epoch as seen
+    by the ledger close clock.
 */
+class Clock // : public abstract_clock <std::chrono::seconds>
+{
+public:
+    using time_point = std::uint32_t;
+    using duration = std::chrono::seconds;
+};
+
+/** A ledger index. */
 // VFALCO TODO pick one. I like Index since its not an abbreviation
 typedef std::uint32_t LedgerIndex;
 // VFALCO NOTE "LedgerSeq" appears in some SQL statement text