scan_coordinator: Add validation for scan parameters

Change-Id: Ibb6f439f2107a266b0ed924fdfca234407d692b9
Reviewed-on: http://review.couchbase.org/43327
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>