MB-18030 Fix a bug that returning the smallest key twice
MB-18030 Fix a bug that returning the smallest key twice

- If iterator's cursor is pointing to the smallest key which exists
in WAL only, we should handle this case separately.

Change-Id: I92174d6f27d6868c6394a514f481cfe867b42245

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