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/shamap/SHAMapSync_test.cpp

@@ -59,7 +59,7 @@ public:
             std::shared_ptr<SHAMapItem> item = makeRandomAS();
             items.push_back(item->key());
 
-            if (!map.addItem(std::move(*item), false, false))
+            if (!map.addItem(SHAMapNodeType::tnACCOUNT_STATE, std::move(*item)))
             {
                 log << "Unable to add item to map\n";
                 return false;
@@ -98,7 +98,8 @@ public:
         int items = 10000;
         for (int i = 0; i < items; ++i)
         {
-            source.addItem(std::move(*makeRandomAS()), false, false);
+            source.addItem(
+                SHAMapNodeType::tnACCOUNT_STATE, std::move(*makeRandomAS()));
             if (i % 100 == 0)
                 source.invariants();
         }