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/main/Application.cpp

@@ -1906,8 +1906,7 @@ ApplicationImp::loadLedgerFromFile(std::string const& name)
             }
         }
 
-        loadLedger->stateMap().flushDirty(
-            hotACCOUNT_NODE, loadLedger->info().seq);
+        loadLedger->stateMap().flushDirty(hotACCOUNT_NODE);
 
         loadLedger->setAccepted(
             closeTime, closeTimeResolution, !closeTimeEstimated, *config_);