Fixed a bug in previous release.

1. Fixed a bug which possibly leads to speed issues in previous release.
2. The last release of this decade.
3. Compacted smux.

$ go version
go version go1.13.5 darwin/amd64

$ date
2019年12月29日 星期日 18时41分08秒 CST
$ ./build-release.sh
Setting GO111MODULE to on
d469e201c9dc269cb513852a2340ae77bef25436  kcptun-linux-amd64-20191229.tar.gz
9e9495dd512a9ad6350e7aa040def03ec2e976dd  kcptun-darwin-amd64-20191229.tar.gz
dbb941cbc7c9d7310550925d34568511ed6d788e  kcptun-windows-amd64-20191229.tar.gz
9626e63d135685a518d5ddf9bca7434ee9dbcbdf  kcptun-freebsd-amd64-20191229.tar.gz
8f88e4f7401ef640b244752aea0d1648b9ed59fc  kcptun-linux-386-20191229.tar.gz
d160fb300a54dec45d23757276fa807e99139696  kcptun-windows-386-20191229.tar.gz
9742db6e82bf3e4d19e0a62615722a70dd4273f1  kcptun-linux-arm5-20191229.tar.gz
2a8d230f0a697ca55a0ea3a26cc7d3b274a00014  kcptun-linux-arm6-20191229.tar.gz
f942f83b30a21ee99acf49655dd14bb3aa4577f9  kcptun-linux-arm7-20191229.tar.gz
0125fed24d82a2acf35db11a5741e5bb50b72088  kcptun-linux-arm64-20191229.tar.gz
58ffcf67d0cddb2f2596cc5016d31068329271ac  kcptun-linux-mipsle-20191229.tar.gz
566cf0082fc32ebafab4f0079dfc64a7103e2431  kcptun-linux-mips-20191229.tar.gz