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.
