feat: expose TCP_NODELAY and TCP_KEEPALIVE

[rapiz1]

• Expose TCP_NODELAY
• Opt-in for each service's data channels
• Set for every control channel
• Expose TCP_KEEPALIVE
• Set default tcp_keepalive_time smaller

Resolves #79

[rapiz1]

@emillynge @juzi5201314 This PR also tweaks trait Transport, but in a mild way. And I expect it will be trivial to cooperate with. Let me know if this is troublesome for you. I will keep this PR open for a day or two and wait for your feedback.

Sorry for the inconvenience. It's hard to shoot a moving target :)

[rapiz1]

@emillynge Do you mind I merge this PR first? I think this PR provides more control over the transport parameters via Transport::hint and QUIC may benefit from it, like enabling users to set the keepalive interval. QUIC can explain hints at need.

[emillynge]

Unfortunately, I don't see it being too useful.

with quinn, settings such as keepalive_interval is set when initializing the connection (in bind and connect). Transport::hint seems to be called after this point, when it cannot be changed anyways.

[rapiz1]

In that case, quic can ignore the hint