Start background level checking even if no initial event is written

[nblumhardt]

For the two-minute background level checks to take place (i.e. in the absence of any other logging), at least one event must reach the Seq sink on start-up. For example, if the initial level is the default Information, an informational or higher event must be logged (it doesn't matter whether Seq actually accepts it).

There are some further subtleties - I think Seq must also have a matching API key configured, and a level associated with it.

This is all intended to prevent unexpected network traffic from the sink, but it's a little surprising, especially now that dynamic level control is a fully-baked feature. The whole background level check needs to be reviewed with an eye to making it "just work" in more cases.

[nblumhardt]

Done