rcache_* calls - flesh out logic and use $MCACHE if available

rcache_* calls - flesh out logic and use $MCACHE if available

 - move from $CFG->enablerecordcache <count> to $CFG->rcache <type>

 - if $CFG->rcache === 'internal' use the cache array
   otherwise, attempt to use $MCACHE

 - implement and use rcache_getforfill() to catch thundering herds

 - the cache lifetime is hardcoded to 2s until we understand better the
   possible race conditions against updates that are not happening via
   update_record() / delete_record()