MB-12030: expose xdcr worker processes setting on UI

Because it's useful knob. I've seen it help with initial rate on LAN
and it should be even more helpful with WAN.

Change-Id: I9ffc914955f7ec26f9e2917f9d4ae3874fe8f863
Reviewed-on: http://review.couchbase.org/40736
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>