dcp_client: Use filter_seqs() method for filtering in get_seqs()

Change-Id: I65168a246eff1b5d064f13299b31505bb3266d72
Reviewed-on: http://review.couchbase.org/40492
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: buildbot <build@couchbase.com>