Faster handling of ACK segments：
The complexity for handling data segment acknowledge changed from O(n) to O(1), good for routers.
xtaci/kcp-go@4624417
xtaci/smux@6cf098d
baf532e
$ go version
go version go1.11.4 darwin/amd64
SHA1SUM
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fb7abe28469fb28155cba99c5b875697855758c0 kcptun-darwin-amd64-20190104.tar.gz
dfea19daf6a158f14d1e81e466c87fcd64df9879 kcptun-darwin-386-20190104.tar.gz
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54b4dc3f72acd6a628633da84344183f983cf3b5 kcptun-freebsd-amd64-20190104.tar.gz
3f22186919255c76063104be7ee30693fc016353 kcptun-freebsd-386-20190104.tar.gz
173a88c5584fecfec9ca47408722b3c1ad5d8189 kcptun-linux-arm-20190104.tar.gz
6881f4befafde7d3d5af17bb12086e048efadc09 kcptun-linux-mipsle-20190104.tar.gz
8b5b1dbd87d7a90706f821e78c6d4dcc2977302d kcptun-linux-mips-20190104.tar.gz