Improve directory insertion & deletion (RIPD-1353, RIPD-1488):

This commit introduces the "SortedDirectories" amendment, which
addresses two distinct issues:

First, it corrects a technical flaw that could, in some edge cases,
prevent an empty intermediate page from being deleted.

Second, it sorts directory entries within a page (other than order
book page entries, which remain strictly FIFO). This makes insert
operations deterministic, instead of pseudo-random and reliant on
temporal ordering.

Lastly, it removes the ability to perform a "soft delete" where
the page number of the item to delete need not be known if the
item is in the first 20 pages, and enforces a maximum limit to
the number of pages that a directory can span.

src/ripple/protocol/impl/Feature.cpp

@@ -113,5 +113,6 @@ uint256 const featureEscrow = *getRegisteredFeature("Escrow");
 uint256 const featureCryptoConditionsSuite = *getRegisteredFeature("CryptoConditionsSuite");
 uint256 const fix1373 = *getRegisteredFeature("fix1373");
 uint256 const featureEnforceInvariants = *getRegisteredFeature("EnforceInvariants");
+uint256 const featureSortedDirectories = *getRegisteredFeature("SortedDirectories");
 
 } // ripple