Improve statistic reporting for shared Rock caches.

Improve statistic reporting for shared Rock caches.

Rock cache is shared between multiple processes.  We need to make sure
cache related stats are not counted twice by different processes.  The
patch makes Rock store statistics to be reported by the disker process
only.

Some global variables for cache related stats are replaced with Store
class methods.  This is needed because it may be difficult or
impossible to correctly update these variables for shared caches.

The patch also fixes cache manager output for some requests like
mgr:storedir: Before the change stats from disker processes were not
surrounded with "by kidN".