Currently live restore can only be configured on startup. We allow live_restore.threads_max to be set to zero on startup so users can run recovery without triggering a full background migration of data, but this assumes the user will at some point enable live_restore to perform migration.

This ticket adds the ability for users to reconfigure threads_max at runtime at which point WiredTiger should update the number of background threads performing migration. If we reconfigure from 0 threads to a non-zero amount this will begin migration.