Add admin API for pausing background updates

[erikjohnston]

If a background update is causing issues it is useful to be able to temporarily pause running of the updates via an admin API.

Open question: should this only pause the BG updates until next restart? We want to avoid the situation where the BG updates get paused and then forgotten about, as that would be a massive footgun

[richvdh]

other things it would be useful for this API to do:

• give a status report on the current bg update situation (raw dump of the background_updates table, contents of the _background_update_performance dict)
• possibly, mess about with the bg update timing params