Enable distributed cache freeing using expire + % of TTL remaining.

When a cache is overly full, we would like to quickly reduce its size. The best way to do so is now to call cache:expire(ServerName, Pct_TTL_Remaining) which will broadcast to all the cache datum pids. Any pid which has less than Pct_TTL_Remaining life will immediately terminate, all others will remain. The 'DOWN' messages cause the cache server size to be updated.