fixed a debug message + minor cleanings and an extra check
ensure returning a correct response if the value is null
don't depend anymore on the main Groupache module to be installed

the client implements the protocol and now uses the standard
Digest::SipHash module to compute the signature
(instead of the facility function exposed by libgroupcache)
so it can be installed to communicate with running instances
without the need of having libgroupcache installed
allow to use/query the Groupcache object from within the run callback

still maintaining thread safety
(since libgroupcache requests will be still served in the background)