Move Rescheduler to an ops charmed library (pip library could be better)

[addyess]

Enhancement Proposal

As part of #118 a library was created from work that was started by @yanksyoon and @cbartz and various others. The library creates a systemd one-shot service and timer which calls back into the charm at a prescribed time outside of the purview of jujud.

This library could be useful to other charm developers since it does a nice job of rescheduling any event rather than using event.defer() while avoiding the drawbacks of event.defer() b/c events can also be cancelled