[entropy.client] always use repositories hashes for cache keys
[entropy.client] remove repositories list cache validation code

Dead code, no longer needed.
[entropy.client] move _enabled_repos to a property, allow lazy loading
[RigoDaemon] use ClientSettings() rather than SystemSettings()[id]
[entropy.client.misc] use ClientSettings() rather than SystemSettings()[id]
[entropy.client.package] use ClientSettings() rather than SystemSettings()[id]
[entropy.client] move ClientSystemSettingsPlugin to a separate module

This will break the circular dependency between loaders and client modules
[client] update entropy.pot
[entropy.output] make the message header variable public
[solo.utils] use the public message header variable
[eit.utils] use the public message header variable