server: misc cleanups

[conradoplg]

Based on #307

This improves some stuff I noticed while working on ZcashFoundation/frost#811

• Change default port of clients to 443 (this will mismatch with the server default 2744 on purpose; the user is supposed to deploy nginx on 443 and redirect to 2744, or do their own thing by specifying the ports using the flags)
• Use PublicKey for other instances of public key in arguments, to make sure they're serialized as hex strings and not as arrays of numbers
• Remove num_signers from session handling, it's not really needed
• Make the server logs in INFO level by default (before it wouldn't log anything unless RUST_LOG variable was set)
• Change tracing logs to DEBUG instead of INFO
• Renamed uuid argument to challenge