CI: yas-run/yas#92 (WAIT CONFLICT race) - #45

A WAIT on a process its session holds no route to looks at it with a temporary
WATCH. That look is refused as CONFLICT while the process's exit is on its way to
its watchers (the record is final only once they all have it), or while the
session's own previous look, such as the one a KILL's CONTROL just made, is still
leaving. So a WAIT right after a KILL of a detached process could answer CONFLICT
instead of the exit: client_host's
a_process_whose_attachment_went_is_waited_for_and_held_by_nothing failed so on a
loaded machine. Such a WAIT now waits for the process catalogue's next change,
with which either settles, and looks again, within its timeout.

a_wait_right_after_a_kill_of_a_detached_process_gets_its_exit (64 rounds of
spawn detachable, detach, KILL, WAIT) failed 1 run in 10 without the change, and
none in 20 with it.

a_watcher_whose_queue_fills_after_the_exit_fails_alone took only output in its
frame loop and panicked on the stdin's progress event, which a slow machine
delivers among the frames; it lets it pass now, and waits for the child's reaping
(Server::wait_reaped, tests only) rather than 500 ms.

A WAIT's temporary look can be refused as CONFLICT because this session's own
look at the process is still bound: a concurrent CONTROL's, or a route that an
eviction failed and has yet to detach (dispatch_outbound fails the route, then
sends the Detach). Such a WAIT waited for the catalogue's next change, which a
Detach doesn't make, so it slept until the process ended and then found nothing
of an ordinary process: NOT_FOUND. It now waits with Manager::wait_native_look,
which also wakes when the record changes and returns once nothing refuses a look:
no exit in flight and no binding of this endpoint.

a_wait_that_finds_its_own_look_still_bound_answers_once_it_goes fails a watcher's
route, WAITs, and detaches the look 200 ms later. Without the change the WAIT
answered NOT_FOUND once the child's second had passed; with it, the exit.

The Exit arm's comment still said such a WAIT answers CONFLICT; it says what it
does now.