dmllib: introducing the rcache_* functions for record cache handling

dmllib: introducing the rcache_* functions for record cache handling

Abstracted Sam's initial work with $record_cache into a series of functions
that abstract things _just enough_ that we can use an internal
in-memory-array implementation or something that is shared across
processes, like memcached or the turckmmcache/eaccelerator caches.

Also
- added hit/miss stats tracking and reporting
- removed max entries limiting as it was buggy - var names mismatches
  and not counting unset()s