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 100% positive, but I would guess yes, it is called from multiple threads.  Here are the places it is called from:

SHAMapNodeID::SHAMapNodeID (int depth, uint256 const& hash);
SHAMapNodeID::getNodeID (int depth, uint256 const& hash);
SHAMapNodeID::selectBranch (uint256 const& hash) const;

I know that selectBranch in particular is called a lot while traversing SHAMaps, and that the SHAMaps are equipped with mutexes to synchronize access.