Remove calls to SHAMapNode::getID

This addresses RIPD-347
Calls in SHAMap::getCache and SHAMap::canonicalize are
purposefully left in place at this time (to be removed
later).
In https://ripplelabs.atlassian.net/browse/RIPD-347 David suggests a new function call descend which helps track the path from the root to any give node.  This path *is* the nodeID.  And the ID of the root node is zero, which is also a default constructed SHAMapNodeID.  I've put the new function in exactly as David suggested, and used it in many places.  It worked quite well.