Clarify Escrow semantics (RIPD-1571):

When creating an escrow, if the `CancelAfter` time is specified but
the `FinishAfter` is not, the resulting escrow can be immediately
completed using `EscrowFinish`. While this behavior is documented,
it is unintuitive and can be confusing for users.

This commit introduces a new fix amendment (fix1571) which prevents
the creation of new Escrow entries that can be finished immediately
and without any requirements.

Once the amendment is activated, creating a new Escrow will require
specifying the `FinishAfter` time explicitly or requires that a
cryptocondition be specified.

src/ripple/protocol/impl/Feature.cpp

@@ -108,7 +108,8 @@ detail::supportedAmendments ()
         { "B9E739B8296B4A1BB29BE990B17D66E21B62A300A909F25AC55C22D6C72E1F9D fix1523" },
         { "1D3463A5891F9E589C5AE839FFAC4A917CE96197098A1EF22304E1BC5B98A454 fix1528" },
         { "F64E1EABBE79D55B3BB82020516CEC2C582A98A6BFE20FBE9BB6A0D233418064 DepositAuth"},
-        { "157D2D480E006395B76F948E3E07A45A05FE10230D88A7993C71F97AE4B1F2D1 Checks"}
+        { "157D2D480E006395B76F948E3E07A45A05FE10230D88A7993C71F97AE4B1F2D1 Checks"},
+        { "7117E2EC2DBF119CA55181D69819F1999ECEE1A0225A7FD2B9ED47940968479C fix1571" }
     };
     return supported;
 }
@@ -158,5 +159,6 @@ uint256 const fix1523 = *getRegisteredFeature("fix1523");
 uint256 const fix1528 = *getRegisteredFeature("fix1528");
 uint256 const featureDepositAuth = *getRegisteredFeature("DepositAuth");
 uint256 const featureChecks = *getRegisteredFeature("Checks");
+uint256 const fix1571 = *getRegisteredFeature("fix1571");
 
 } // ripple