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).
Not disagreeing, but how does this work?  We used to ask the root to give us a NodeID in the call to replace.  Now we're using a default constructed NodeId in that same place.  Casual inspection suggests that we've lost information.  Does the information in rootID get filled in by the call to make_shared?  If that's the case it seems like an unexpected side effect.