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).
I think I've got it.  Most of the cases that were most puzzling to me involved the root node.  It turns out that a default constructed SHAMapNodeID always corresponds to the SMAMap root node.  Presumably because the ID of the root node is always all zeros.