MB-23750: Fix toggling ephemeral ejection policy

MB-23750: Fix toggling ephemeral ejection policy

If going

auto_delete -> fail_new_data -> auto_delete

the itemPager task was rescheduled, but as it had previously been
cancelled the state remained TASK_DEAD, and so ExecutorThread::run
behaves as if the task has just been cancelled and needs cleaning up.
By resetting scheduled tasks back to TASK_RUNNING if they are dead, this
can be avoided.

Change-Id: Id007a15fdaeb80a79828da0cade031a424e653cf
Reviewed-on: http://review.couchbase.org/76394
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>