* Alerting: Refactor `Run` of the scheduler
A bit of a refactor to make the diff easier to read for supporting
external Alertmanagers.
We'll introduce another routine that checks the database for
configuration and spawns other routines accordingly.
* Block the wait.
* Fix test
(cherry picked from commit 442a6677fc)
Co-authored-by: gotjosh <josue@grafana.com>