MB-18747: short-circuit _wal_can_discard if WAL not shared
MB-18747: short-circuit _wal_can_discard if WAL not shared

Change-Id: I1d2780021d584d3c41dd6c79c5c077ba38b98cfc
Reviewed-on: http://review.couchbase.org/61952
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>

wurikiji/forestdb,hisundar/forestdb,wurikiji/forestdb,couchbase/forestdb,couchbase/forestdb,hisundar/forestdb