client: increase finish-file timeout

[davidpanderson]

When an app finishes, it writes a "finish file",
which ensures the client that the app really finished.

If the app process is still there N seconds after the finish file appears,
the client assumes that something went wrong, and it aborts the job.

Previously N was 10.
This was too small during periods of heavy paging.
I increased it to 300.

It has been pointed out that if the app creates the finish file,
and its output files are present,
it should be treated as successful regardless of whether it exits.
This is probably true, but right now we don't have a mechanism
for killing a job and marking it as success.
The longer timeout makes this moot.

Fixes #3017

[JuhaSointusalo]

It has been pointed out that if the app creates the finish file,
and its output files are present,
it should be treated as successful regardless of whether it exits.

FWIW, it has also been argued that if an app hangs at exit there's no trusting anything that happened before the hang. The context in that discussion was GPU apps. If GPU drivers cause an app to hang at exit then the drivers might not have processed computing commands correctly either.