MB-9695: idle cpu usage reduction optimizations

optimize scheduler's fetchNextTask loop
reduce sleeptime in tapconnmap

Change-Id: If90eda0b862527b7f8d3d78a8dd5022d02394490
Reviewed-on: http://review.couchbase.org/31008
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>