Refactor table scan in couchkvstore

We no longer use the code that allows us to schedule a table scan
for all vbuckets. This code should be especially removed since we
have multiple couchkvstore instances per bucket and doing a table
scan for all vbucket means all 1024 vbucket.

Change-Id: Ib0aee0ca9621a123ee0d46906448c9a08bd72dc0
Reviewed-on: http://review.couchbase.org/30762
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>