Packages: cache group and package selections
Plugin: added new Caching interface

This gives a single unified interface for expiring caches, no matter
the plugin.  This will be particularly useful with the
MultiprocessingCore, as certain calls must be dispatched to child
processes to expire their caches.
MultiprocessingCore: added a way to dispatch RMI calls to child processes
Plugin: added missing docstring
Metadata: catch dns lookup errors better
testsuite: perform pylint tests against MultiprocessingCore when appropriate
Reporting: Fix typo

Signed-off-by: Sol Jerome <sol.jerome@gmail.com>