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/overlay/impl/Message.cpp

@@ -177,6 +177,12 @@ Message::setHeader(
     }
 }
 
+std::size_t
+Message::getBufferSize()
+{
+    return buffer_.size();
+}
+
 std::vector<uint8_t> const&
 Message::getBuffer(Compressed tryCompressed)
 {