Remove unnecessary lock from Task class.

Task::snooze is always invoked by only one thread at any time.

Change-Id: If0921eadafb562cd124d8ce29b154f47f77861c5
Reviewed-on: http://review.couchbase.org/40042
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>