[entropy.client] Client: move object attributes to @property to enable lazy loading
[entropy.client] use ClientSettings() whenever needed

This commit also drops the sys_settings_client_plugin_id attribute
[entropy.client] lazy load ClientSystemSettingsPlugin
[entropy.client] do not clear cache if xcache is False

Nowadays, cache in entropy.client uses hashes of repositories
as part of the key, there is no need to clear cache explicitly