[entropy.db.cache] introduce EntropyRepositoryCachePolicies

It is now possible to define a policy for the in-RAM cache through
EntropyRepository constructor or EntropyRepositoryCachePolicies.
[entropy.db] expose and use the currently set cache policy
[RigoDaemon] set the cache policy to EntropyRepositoryCachePolicies.NONE

This greatly reduces the memory consumption of RigoDaemon
[entropy.tools] add total_memory() function
[entropy.db.cache] disable in-RAM cache if total memory is less than 4G
[Rigo] fix NotificationBox message_type keyword argument
[entropy.spm] PortagePlugin: update portage.doebuild() call, remove deprecated root argument
Release Entropy 262
Tagging Entropy version 262
[entropy.db.sqlite] do not acquire the repo lock if it's not strictly needed

Try to avoid to acquire the repository lock in exclusive mode if
it's not strictly needed. We were acquiring the lock in exclusive mode
even when there was nothing to update due to schema changes.
Release Entropy 263
Tagging Entropy version 263
[Rigo] Update the Slovak translation, thanks to Dušan Kazik
[entropy.core] SystemSettings: add lazy loading of metadata
[entropy.core] SystemSettings: lazy load item only if it hasn't been loaded yet
[entropy.core] SystemSettings: simplify __getitem__
[client] update Slovak translation, thanks to Dušan Kazik
[client] update translations
[entropy.core] SystemSettings: __remove_repo_cache() is no longer necessary
[entropy.core] SystemSettings: drop mtime based cache validation

This is no longer necessary and error prone on filesystems with
coarse mtime granularity (like ext3). Individual methods are able
to validate their own caches.