Option to disable logging of client ips

[abeluck]

Currently, shadowsocks-go will write the client ip address to the log if a connection fails:

Feb 05 14:45:49 shadowsocks-test-ansible shadowsocks-server[30172]: 2018/02/05 14:45:49 error getting request client.ip:port 10.0.2.15:443 read tcp 10.0.2.15:443->client.ip:port:43000: read: 
connection reset by peer

Ideally there would be an option to disable this for privacy-conscious services who need to keep logs for debugging/perf monitoring but do not want to store client ips.

[abeluck]

@arthurkiller I will create a PR for this. Do you want it submitted against develop or master? Seems like develop is far behind master.

[arthurkiller]

FIXED