Give each check a timeout by default

Ok, so this is an exercise in sane defaults.  Here are the ones I've chosen; let me know what you think:

* a check should time out after its interval has elapsed.  I considered 50% * interval, but I think that might be too strict.
* the maximum chosen timeout for any check should be one hour.
* if necessary, a user can manually provide a timeout.
Good to see that this is a "normal" parameter in the normal type