swap another locked CMPXCHG for XCHG

Now instead of finding the first available slot to put the migrating pointer in,
we pick the slot it immediately hashes to and shuffle all the other elements
along one. In theory this should be more efficient based on an XCHG being
cheaper than locked CMPXCHG.

Github: closes #179 "replace cmpxchg with xchg during set migration"