Allow a per-jobset check interval

This allows checking a jobset (say) at most once a day.  It's also
possible to disable polling by setting the interval to 0.  This is
useful for jobsets that use push notification or are manually
evaluated.

src/sql/upgrade-13.sql

@@ -0,0 +1 @@
+alter table Jobsets add column checkInterval integer not null default 300;