optimizations on -tcp option for linux

1. support -tcp option for linux(EXPERIMENTAL) . To enable tcp transmission, you should enable it on both client and server, supported only under LINUX with iptables, require root privilege.

2. fix a potential race between cleaner() goroutine and WriteTo()

discussion on this release:
#696

$ go version
go version go1.12.7 linux/amd64

$ uname -a
Linux DESKTOP-7FL7RC4 4.4.0-18362-Microsoft #1-Microsoft Mon Mar 18 12:02:00 PST 2019 x86_64 x86_64 x86_64 GNU/Linux

$ shasum *.tar.gz
763b43bce8d34c586af0884794d7aac7e2e9bf0d  kcptun-darwin-amd64-20190717.tar.gz
db735672cb10b5a2958d52429bc7afab175b047e  kcptun-freebsd-amd64-20190717.tar.gz
c95d9bb6117289b195401fc2e1b6a1d2ae7a3828  kcptun-linux-386-20190717.tar.gz
834498340bfcf6d3bcaa1cb3b2d9dd6b969353fb  kcptun-linux-amd64-20190717.tar.gz
de131900a1d7efb5299d7050c68a7877d721042c  kcptun-linux-arm5-20190717.tar.gz
34b15b668bc1c20d11a3545f8b8d53900796e3b7  kcptun-linux-arm6-20190717.tar.gz
3ca2abec040c11af1c59ddcb45c23e90840006a0  kcptun-linux-arm64-20190717.tar.gz
5d481cbe3e09009056e8c9bf2443cd3ae2995e10  kcptun-linux-arm7-20190717.tar.gz
53e030e7594905cba846d2a271035f6fe757f478  kcptun-linux-mips-20190717.tar.gz
3d2449b1b56be927446edd6a3aaabb1523552fd1  kcptun-linux-mipsle-20190717.tar.gz
ad92a7f49ac94d89583b32a8b874b9e68532b129  kcptun-windows-386-20190717.tar.gz
0bf62af3a6af3ea271310dbc57ad9679f94bcb4d  kcptun-windows-amd64-20190717.tar.gz

$ date
Wed Jul 17 15:55:36 CST 2019