Refactor get_sequence_number/1

Create a get_sequence_number/2 that can be called without a gen_server
call in case the setname is already available.

This change is needed for a subsequent commit where you need to get
the sequence number within a gen_server:handle_call/3.

Change-Id: I83fb1166daf569e6c66de1fd43d67f3f8cecc931
Reviewed-on: http://review.couchbase.org/34434
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Volker Mische <volker.mische@gmail.com>