Implement new amendment majority semantics :

This implements the tracking of when an amendment achieved a majority
in the ledger, ensuring that there's always network-wide agreement
on which amendments have achieved a majority and how long they've
held it.

* New fields
* Change transactor changes
* AmendmentTable API and implementation changes
* Update amendment enabled status on validated ledgers
* Reinstate support for ledger sequence in fee transactions

src/ripple/ledger/View.h

@@ -131,6 +131,16 @@ cdirNext (ReadView const& view,
     unsigned int& uDirEntry,    // <-> next entry
     uint256& uEntryIndex);      // <-- The entry, if available. Otherwise, zero.
 
+// Return the list of enabled amendments
+using enabledAmendments_t = std::set <uint256>;
+enabledAmendments_t
+getEnabledAmendments (ReadView const& view);
+
+// Return a map of amendments that have achieved majority
+using majorityAmendments_t = std::map <uint256, std::uint32_t>;
+majorityAmendments_t
+getMajorityAmendments (ReadView const& view);
+
 //------------------------------------------------------------------------------
 //
 // Modifiers