Refactor and improve the SHAMap code:

This commit combines a number of cleanups, targeting both the
code structure and the code logic. Large changes include:

 - Using more strongly-typed classes for SHAMap nodes, instead of relying
   on runtime-time detection of class types. This change saves 16 bytes
   of memory per node.
 - Improving the interface of SHAMap::addGiveItem and SHAMap::addItem to
   avoid the need for passing two bool arguments.
 - Documenting the "copy-on-write" semantics that SHAMap uses to
   efficiently track changes in individual nodes.
 - Removing unused code and simplifying several APIs.
 - Improving function naming.

src/ripple/app/misc/SHAMapStoreImp.cpp

@@ -299,12 +299,10 @@ SHAMapStoreImp::fdRequired() const
 }
 
 bool
-SHAMapStoreImp::copyNode(
-    std::uint64_t& nodeCount,
-    SHAMapAbstractNode const& node)
+SHAMapStoreImp::copyNode(std::uint64_t& nodeCount, SHAMapTreeNode const& node)
 {
     // Copy a single record from node to dbRotating_
-    dbRotating_->fetchNodeObject(node.getNodeHash().as_uint256());
+    dbRotating_->fetchNodeObject(node.getHash().as_uint256());
     if (!(++nodeCount % checkHealthInterval_))
     {
         if (health())