Extract out the schedule monitoring from Schedule class

[pdewouters]

The HMBKP_Scheduled_Backup class has too many responsibilities.

One functionality we can abstract out is the status monitoring.

Schedules could publish status to subscribed objects using the Observer pattern. We could have a single Backup_Monitor class or whatever that would have a collection of all running backups, and handle status changes. It would maybe make it easier to have parallel running backups with Backdrop.

Why are we writing status to a file? Could it not be a DB option?

Ha, actually suggested by Scrutinizer
https://scrutinizer-ci.com/g/humanmade/backupwordpress/code-structure/master/class/%2Bglobal%5CHMBKP_Scheduled_Backup

Thoughts? @willmot

[willmot]

Huge +1

[willmot]

Feel the same about HM_Backup

[willmot]

Also see #667.

Why are we writing status to a file? Could it not be a DB option?

Because DB connections can go away, generally the FS is more reliable.

[willmot]

Done as part of #936