Cancel in-progress connection attempts (fixes #58)

This fixes a problem (#58) where we wouldn't close connections if they weren't fully open. If the connection attempt was still in progress when we requested a shutdown, the connection attempt might complete and never shut down, which could lead hangs when shutting down a `PushManager` without a timeout.

Note that this can sometimes produce warnings from Netty; that problem is fixed in 4.0.18, which ~~I'm hoping will be out soon~~ is out now and added in 3000226.