[xdcr] refactored capi_replication checkpointing params parsing

Particularly extracted common code between extract_ck_params and
handle_mass_vbopaque_check. And also made it respond with 400 on
missing params rather then 404 which we used to do.

Change-Id: Iac11952590e7e8df0c4a1b1cf18a6f5ea9ffa729
Reviewed-on: http://review.couchbase.org/40384
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>