* Fix HTTP1 to HTTP2 migration while shutdown is in progress
### Motivation
Calling `HTTPClient.shutdown()` may never return if connections are still starting and one new established connection results in a state migration (i.e. from HTTP1 to HTTP2 or vice versa). We forgot to migrate the shutdown state. This could result in a large dealy until `.shutdown()` returns because we wait until connections are closed because of idle timeout. Worse, it could also never return if more requests are queued because the connections would not be idle and therefore not close itself.
###Changes
- Mirgrate shutdown state too
- add tests for this specific case
* simplify testMigrationFromHTTP1ToHTTP2WhileShuttingDown
* add http2 to http1 migration test
If a request times out because no connection could be attained, we should fail the request with the last connection creation error. If no connection creation error is present and there is no active connection, we fail the request with a connectTimeout error.
David Nadoba
Fabian Fett