Make arch_updates module more responsive to system updates

[geoffliu]

The goal of this PR is that we want to invalidate the cached output of the arch_updates module after a system update. The gist of the approach is this:

• remove cached_until and cache_timeout values, thus bypassing built-in caching of py3status
• use module storage as a cache
• if pacman.log has been updated, invalidate cache

This whole business of bypassing the built-in caching feels a bit dirty, but I'm not familiar with py3status's full module API to know if there's a better approach? @ultrabug What do you think?

Also, now that the module can be more responsive, we can change the default of refresh interval to be longer without degrading user experience

[ultrabug]

I trust you as a module user and the logic LGTM thanks!