Add V2 implementation of payments:

Add a new algorithm for finding the liquidity in a payment path. There
is still a reverse and forward pass, but the forward pass starts at the
limiting step rather than the payment source. This insures the limiting
step is completely consumed rather than potentially leaving a 'dust'
amount in the forward pass.

Each step in a payment is either a book step, a direct step (account to
account step), or an xrp endpoint. Each step in the existing
implementation is a triple, where each element in the triple is either
an account of a book, for a total of eight step types.

Since accounts are considered in pairs, rather than triples, transfer
fees are handled differently. In V1 of payments, in the payment path
A -> gw ->B, if A redeems to gw, and gw issues to B, a transfer fee is
changed. In the new code, a transfer fee is changed even if A issues to
gw.

src/ripple/protocol/Feature.h

@@ -39,6 +39,7 @@ extern uint256 const featureTickets;
 extern uint256 const featureSusPay;
 extern uint256 const featureTrustSetAuth;
 extern uint256 const featureFeeEscalation;
+extern uint256 const featureFlowV2;
 
 } // ripple