Any time the state of the check changes, we need to refresh the column.
This will be used in the frontend. This way it is much faster than using
the notifications and also doesn't rely on the last of it being still
around.
It happend all the time, that connections were hanging in idle in
transaction state.
This was caused by a rollback stuck behind a sleep loop to wait for the
next bunch of checks to run.
This also replaces a Query() call with QueryRow, as we only expect a
single row and this cleans up the code a bit.
The default worker count was set to 25, which might be too much
depending on the amount of cores available. Now make the default 25 and
let people decide on how many they want to use.
It would be nice to support multiple notification mechanisms with a way
to disable all or part of them. So for now, this is bound to the check
but may change again in the future.
Apart from that, all notifications get written to the notification
table, so that the notifiers can do their job and ACK their own
notifications.
This adds mapping levels to the check runner. When the exit code is
returned, the actual level is looked up using the configured mapping.
After that, the state is entered into the table and added the same way
to the notifications.
