xdcr: configure init throttle with at least one token

Otherwise paused replication causes us to display 0 in all stats after
node startup.

Change-Id: Iee3dac548e6db2ac2ebec59e88cae6fa890068d7
Reviewed-on: http://review.couchbase.org/40509
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>