M. without locking

Should insert a speculative entry before starting. If it finds a speculative entry already there, wait for it to turn into an actual entry, so as to avoid duplicating work across threads. Can adapt cliff click 'non blocking hash map' for the implementation, though hayley patton says it is faster to lock on resize than use his non-blocking resize.

(And use hazard pointers for the resize. I have most of an implementation somewhere, adapted from paul khuong.)