Use static lambdas.

[bartdesmet]

Check if the CI build does already support C# 9.0 features. If so, conversion to static lambdas (to help identify cases where we have unexpected or avoidable closures) can continue.

[bartdesmet]

First pass over a subset of the code, while addressing a few unnecessary closures in the process. Future PRs will continue this work:

1. Mark more existing lambdas as static where possible.
2. Try to convert closure-capturing lambdas to static where possible and worthwhile.
3. Optionally annotating the non-static lambdas with a clear indicator of a closure (/* closure */) so these can be easily CTRL-F'd.

[bartdesmet]

Possible flaky test to be investigated:

EventLoop_ScheduleTimeAndOrderedInFlightActions