Remove the lock around NameCache for readBinIface.

Remove the lock around NameCache for readBinIface.

Turns out using atomic update instead of a full-blown lock was easier
than I thought.  It should also be safe in the case where we
concurrently read the same interface file.  Whichever thread loses the
race will simply find that all of the names are already defined and
will have no effect on the name cache.