[entropy.db.skel] expose atomMatch cache key generation hook

This method adds atomMatchCacheKey() that can be reimplemented
by subclasses in order to cooperatively generate a cache key for
atomMatch calls.
[entropy.client.db] stop using EntropyCacher.CACHE_IDS['mask_filter']

Use atomMatchCacheKey() instead
[entropy.client.methods] drop _clear_repository_cache() usage

This is no longer needed because cache keys are now properly generated
[entropy.core.settings.base] add license_accept and system_mask to packages_configuration_hash()
[entropy.core.settings.base] drop _clear_repository_cache(), no longer needed
[entropy.core.settings.base] drop any explicit clear_cache() call

clear_cache() is no longer required since cache keys are now
correctly generated.
[entropy.cache] EntropyCacher: drop CACHE_IDS (\o/) and clear_cache()
[todo] update TODO