Ensure that all dbt invocations have an invocation_id

[gshank]

This is a sub-issue of #4260.

In order to make sure we can distinguish logs from different runs we should always have an invocation_id. If tracking is off, it isn't created today so we would need to create it. (to add it to logging, add it as a class property on the superclass) (previously considered: preserve process id like it is today, global run_id reported at start, or allow some sort of user markers).