1.0.15.12: better scaling in the PCL cache

1.0.15.12: better scaling in the PCL cache

 * When the cache reaches its maximum size, and entries need to be
   dropped, drop a random 50% of them, instead of the more
   deterministic set "ones that don't fit": this avoids getting stuck
   in a "add A dropping B, add B dropping A, ..." cycle which eats up
   ginormous amounts of time. Additionally, dropping 50% seems to be
   the best ratio -- experimentally, at least -- but it would be nice
   to have a proper analysis...

   Note: there is a point (possibly even before our current maximum
   cache size) where the allowed probe-depth grows so large that a
   tree would work better then a table. It would be good to gracefully
   replace the table based cache with a tree when it grows so large.