MB-7217: allow lowering xdcr checkpointing interval to 10 seconds

Change-Id: Ia3b72e2035b059ae21c0ec0d53411c06a02b563a
Reviewed-on: http://review.couchbase.org/40482
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>