This PR improves stability and user experience by terminating, instead of waiting indefinitely, in the following cases:

Kill connections in the shared::proxy() function when either duplex stream direction terminates, rather than both.

For example, if a user starts a connection at bore.pub:12345 that forwards a raw HTTP/2 connection to their localhost port, and they kill the web server at that port, then previously future HTTP/2 requests in the same browser session to bore.pub:12345 would hang indefinitely because they would reuse the same TCP stream. With this change, the TCP stream would be closed immediately instead.

Adds a timeout to initial protocol messages.

Initial protocol messages should be expected immediately, so if they are not sent, that indicates a network partition or other issue. This adds a timeout to such messages. It catches cases such as when the -s flag is passed to bore local even when the server is configured to not use a secret. Previously this case would wait indefinitely for a server challenge.

Adds a timeout to initial TCP connections.

This fixes issues such as typo in the TCP connection string. Previously running something like bore local 5000 --to google.com would wait indefinitely for a connection to google.com:7835. Now it terminates gracefully with an error.