MB-9495: notify threads on task wake to avoid loss

To avoid a lost post from the pthread's signal routine
notify all threads on a task wake

This change benefits not only bg fetch task but also flusher

Change-Id: I1ad37d11dc9b256bd4b88a1b6860fbf10d22cfcf
Reviewed-on: http://review.couchbase.org/30196
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>