Add set_nodelay and set_keepalive

[eataix]

These additions are similar to those in 271e24d, which were then
removed during the transition to futures and tokio in 5bf6560.

[zonyitoo]

I don't think it is a good idea to set_nodelay. Because we uses streaming protocol assembling, which means that we may call send multiple times while assembling a single protocol header (shadowsocks' protocol header).

[sorz]

+1 for TCP keepalive
This should be the correct implementation for dead connection detection.
"copy timeout" may better to be disable or have a large value (like dozens hours).