Pack the binheap into a structure which will allow us to have
multiple instances at a later date.

Put the "LRU_DONTMOVE" flag on the lru, rather than the objcore.
Move inserts into the binheap into the expiry-thread.

This should make object creation faster.

As a sideeffect, we loose the $expiry_sleep parameter:  Now we always
sleep optimally.