MB-10027: use GlobalTask in flusher to avoid pool mutex

This fix saves reduces the number of lock contentions
in the write path and improves flusher performance as
seen in code profiling report analysis

Use GlobalTask * pointer directly instead of ExTask to
avoid reference counting when there is no need

Change-Id: I39246bdc97e19d9e9bc974e99dab61bd4091ba4d
Reviewed-on: http://review.couchbase.org/35900
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>