Support for lsfDepositAuth (RIPD-1487):

The DepositAuth feature allows an account to require that
it signs for any funds that are deposited to the account.
For the time being this limits the account to accepting
only XRP, although there are plans to allow IOU payments
in the future.

The lsfDepositAuth protections are not extended to offers.
If an account creates an offer it is in effect saying, “I
will accept funds from anyone who takes this offer.”
Therefore, the typical user of the lsfDepositAuth flag
will choose never to create any offers.  But they can if
they so choose.

The DepositAuth feature leaves a small gap in its
protections.  An XRP payment is allowed to a destination
account with the lsfDepositAuth flag set if:

- The Destination XRP balance is less than or equal to
  the base reserve and

- The value of the XRP Payment is less than or equal to
  the base reserve.

This exception is intended to make it impossible for an
account to wedge itself by spending all of its XRP on fees
and leave itself unable to pay the fee to get more XRP.

This commit

- adds featureDepositAuth,

- adds the lsfDepositAuth flag,

- adds support for lsfDepositAuth in SetAccount.cpp

- adds support in Payment.cpp for rejecting payments that
  don't meet the lsfDepositAuth requirements,

- adds unit tests for Payment transactions to an an account
  with lsfDepositAuth set.

- adds Escrow and PayChan support for lsfDepositAuth along
  with as unit tests.

src/ripple/protocol/Feature.h

@@ -73,7 +73,8 @@ class FeatureCollections
         "fix1512",
         "fix1513",
         "fix1523",
-        "fix1528"
+        "fix1528",
+        "DepositAuth"
     };
 
     std::vector<uint256> features;
@@ -353,6 +354,7 @@ extern uint256 const fix1512;
 extern uint256 const fix1513;
 extern uint256 const fix1523;
 extern uint256 const fix1528;
+extern uint256 const featureDepositAuth;
 
 } // ripple