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/test/app/LedgerHistory_test.cpp

@@ -72,8 +72,8 @@ public:
         res->updateSkipList();
 
         {
-            res->stateMap().flushDirty(hotACCOUNT_NODE, res->info().seq);
-            res->txMap().flushDirty(hotTRANSACTION_NODE, res->info().seq);
+            res->stateMap().flushDirty(hotACCOUNT_NODE);
+            res->txMap().flushDirty(hotTRANSACTION_NODE);
         }
         res->unshare();