Implement connection attempt exponential timeout

This is not for exponential backoff, but for exponential timeout. Currently the timeouts involved aren't tuned at all between attempts. We want to have a single timeout applying to all parts of the connection attempt, as defined in the [connection backoff spec](https://github.com/grpc/grpc/blob/master/doc/connection-backoff.md). This generally prevents the need for users to specify connect timeouts.