Improve client usability

[Octav]

The client should be usable without defining a new instance for each new host:port. The logic from the client for connection reusability should be moved inside HTTP2ClientConnection.

[Vlad Stefan Munteanu]

Once this is implemented, we can go back to only one instance of client per ioloop, as tornado does it.
We could look into using tornado's AsyncHTTPClient as a base class and simplify our client.