Expose h2's server Connection::has_streams()

[arthurdandrea]

Expose h2's server Connection::has_streams() introduced in hyperium/h2#838, allowing external keepalive timeout management.

Since hyper's Connection does not have a h2 Connection during handshake, I've exposed a Connection::has_handshake_completed(), but I'm not sure if this is the best approach, maybe returning false on has_streams() is good enough, or even returning Option from the has_streams() method.