Add config settings for background update parameters

[erikjohnston]

It is useful to be able to slow down or speed up background updates, so we should make the targets configurable. Including: target duration, sleep times, min batch size and default batch size.

[richvdh]

see also #11401 (comment): I think we should reduce the default for MINIMUM_BACKGROUND_BATCH_SIZE to 1.

[DMRobertson]

A module exists for this which makes life easier for EMS, but we still think this is worth doing.

[H-Shay]

Fixed by #11980.