Change job mode of manager triggered restarts to JOB_REPLACE

Fixes: #11305
Fixes: #3260
Related: #11456

So, here's what happens in the described scenario in #11305. A unit goes
down, and that triggeres stop jobs for the other two units as they were
bound to it. Now, the timer for manager triggered restarts kicks in and
schedules a restart job with the JOB_FAIL job mode. This means there is
a stop job installed on those units, and now due to them being bound to
us they also get a restart job enqueued. This however is a conflicts, as
neither stop can merge into restart, nor restart into stop. However,
restart should be able to replace stop in any case. If the stop
procedure is ongoing, it can cancel the stop job, install itself, and
then after reaching dead finish and convert itself to a start job.
However, if we increase the timer, then it can always take those units
from inactive -> auto-restart.

We change the job mode to JOB_REPLACE so the restart job cancels the
stop job and installs itself.

Also, the original bug could be worked around by bumping RestartSec= to
avoid the conflicting.

This doesn't seem to be something that is going to break uses. That is
because for those who already had it working, there must have never been
conflicting jobs, as that would result in a desctructive transaction by
virtue of the job mode used.

After this change, the test case is able to work nicely without issues.

(cherry picked from commit 03ff2dc7)
(cherry picked from commit 677b4cc7)