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/impl/Feature.cpp

@@ -106,7 +106,8 @@ detail::supportedAmendments ()
         { "6C92211186613F9647A89DFFBAB8F94C99D4C7E956D495270789128569177DA1 fix1512" },
         { "67A34F2CF55BFC0F93AACD5B281413176FEE195269FA6D95219A2DF738671172 fix1513" },
         { "B9E739B8296B4A1BB29BE990B17D66E21B62A300A909F25AC55C22D6C72E1F9D fix1523" },
-        { "1D3463A5891F9E589C5AE839FFAC4A917CE96197098A1EF22304E1BC5B98A454 fix1528" }
+        { "1D3463A5891F9E589C5AE839FFAC4A917CE96197098A1EF22304E1BC5B98A454 fix1528" },
+        { "F64E1EABBE79D55B3BB82020516CEC2C582A98A6BFE20FBE9BB6A0D233418064 DepositAuth"}
     };
     return supported;
 }
@@ -154,5 +155,6 @@ uint256 const fix1512 = *getRegisteredFeature("fix1512");
 uint256 const fix1513 = *getRegisteredFeature("fix1513");
 uint256 const fix1523 = *getRegisteredFeature("fix1523");
 uint256 const fix1528 = *getRegisteredFeature("fix1528");
+uint256 const featureDepositAuth = *getRegisteredFeature("DepositAuth");
 
 } // ripple