Make wormhole server accept multiple tunnel protocols

[mbyczkowski]

Right now the wormhole server is configured to accept one tunnel protocol (e.g. TCP/TLS conn pool or SSH). Given that we're working on adding HTTP2 tunneling (#8), it would be nice if various clients could just connect to the server.

Client-side:

• can connect to the server and pass config info
• display error/warning info for unsupported config
• could include auth to check if token is valid

Server-side:

• validate client request
• provide endpoint info for client to connect to (e.g. SSH server, HTTP2 server, TLS server)
• could check for client versions to determine if they're still supported.

[doodles526]

being addressed at https://github.com/doodles526/wormhole/tree/handler-rewrite