s4/torture: add a test for ctdb-tombstrone-record deadlock

s4/torture: add a test for ctdb-tombstrone-record deadlock

This tests for a possible deadlock between smbd and ctdb dealing with
ctdb tombstone records.

Commit 925625b52886d40b50fc631bad8bdc81970f7598 explains the deadlock in
more details and contains the fix. It's a fix for a regression
introduced by the patch for bug 10008 (1cae59ce112c).

If you ever want to use this test against that specific commit:

$ git checkout 925625b52886d40b50fc631bad8bdc81970f7598
$ git cherry-pick THIS_COMMIT

This should not deadlock on a ctdb cluster.

$ git revert 925625b52886d40b50fc631bad8bdc81970f7598

This will deadlock.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=12005

Pair-Programmed-With: Michael Adam <obnox@samba.org>

Signed-off-by: Ralph Boehme <slow@samba.org>
Signed-off-by: Michael Adam <obnox@samba.org>