Add support for service timers

[niemeyer]

We’ll start working on this right after we fix the monthly support for refresh scheduling, and we’ll use the same syntax on both. Please have a look at the current syntax proposal if you’re interested in timer services:

Refresh scheduling on specific days of the month

After some extensive and independent bikeshedding on the syntax, what follows below is a proposal for how the new syntax might look like and behave. But first, the high-level goals that the proposal attempts to achieve: Goals Good readability, although some initial familiarity may be required Good usability in the command line (no escaping, no quoting) Support for time windows rather than just individual times Support for fuzzing inside time windows Support for terse and readable recurring ev…