MB-22256: Fix intermittent failures in test_duplicate_items_disk

MB-22256: Fix intermittent failures in test_duplicate_items_disk

The ep_testsuite: test_duplicate_items_disk test occassionally fails
because waiting for the ep_vbucket_del stat to update (from 0 to 1)
times-out.

The reason the stat occasionally does not increase is because the
the vbucket that is undergoing deletion is set back to active before
the deletion is complete.

A side-effect of setting a vbucket to active, is that if the vbucket
does not exist then it is recreated.

This patch moves the setting of the vbucket back to active to after we
have confirmed the deletion has completed (by ensuring the
ep_vbucket_del stat increases).  Thereby removing the race condition.

Change-Id: Ied680c64536862829974fd061a25d317f766bb46
Reviewed-on: http://review.couchbase.org/73758
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Jim Walker <jim@couchbase.com>
Reviewed-by: Manu Dhundi <manu@couchbase.com>