[PATCH] Ignore and warn about attempts to reconfigure static Rock store options.

Hi.

Attached patch does some polishing for Rock store cache_dir
reconfiguration.

Some Rock store options cannot be changed dynamically: path, size, and
max-size. Before the change, there were no checks during reconfigure
to prevent changing these options. This may lead to Rock cache
corruption and other bugs. The patch adds necessary checks to Rock
store code. If user tries to change an option that cannot be updated
dynamically, a warning is reported and the value is left unchanged.

Regards,
  Dmitry