Some progress on the pcp::cache::lookup signatures.
Switched the requireActive boolean with a generic lookup_flags type, for future expansion.
Removed the @todo comment for something that was just done.
Implemented error handling in the pcp::cache::purge function(s).
Implemented error handling in the pcp::cache::perform function.
Implemented error handling in the pcp::cache::store functions.
Some pcp::cache::store function signature corrections.
Simplified the cache operations in the 'simple' PMDA example, using the latest pcp::cache functions.