Add a control interface for BackgroundActivityWorkers

[prioux]

Currently, the number of worker processes that handle BackgroundActivities are hardcoded at boot time.

A portal starts three (3) such workers, a Bourreau starts one (1).

I'd like an interface somewhere to allow the admin to change that.

Also, I'd like an interface to start and stop them. Currently, a TERM signal will make a worker finish whatever it is doing (if any) and then stop cleanly.

The interface should exist at two levels: 1. within the web admin interface 2. in the console's command interface (maybe through 'ibc'?).