Awaiting meta-data providers after checking state in dispatchCall

[jonasfj]

We are awaiting meta-data providers here:

https://github.com/grpc/grpc-dart/blob/054ac9de3c86941dedfe1c6896e0e9df8375c399/lib/src/client/call.dart#L149-L155

This happens after we've check the state of the connection in dispatchCall:

https://github.com/grpc/grpc-dart/blob/054ac9de3c86941dedfe1c6896e0e9df8375c399/lib/src/client/http2_connection.dart#L109-L123

Thus, the state of the connection may have changed, this could cause us to attempt to make a call on a connection that is broken.