A periodic cache warmer.

Child specification options available:

• cache_impl - module expected to implement Plausible.Cache behaviour
• interval - the number of milliseconds for each warm-up cycle
• cache_name - defaults to cache_impl.name() but can be overridden for testing
• force_start? - enforcess process startup for testing, even if it's barred by Plausible.Cache.enabled?. This is useful for avoiding issues with DB ownership and async tests.
• warmer_fn - by convention, either :refresh_all or :refresh_updated_recently, both are automatically provided by cache_impl module. Technically any exported or captured function will work, if need be.

See tests for more comprehensive examples.