* Alerting: Add GetFullState method to FileStore
* make tests compile, create stateStore in NewAlertmanager
* return errors instead of logging, accept an arbitrary number of strings
* make NewAlertmanager() accept a stateStore
* Alerting: Persist notification log and silences to the database
This removes the dependency of having persistent disk to run grafana alerting. Instead of regularly flushing the notification log and silences to disk we now flush the binary content of those files to the database encoded as a base64 string.
