Limit max RTO backoff to 8xRTT.
限定RTO退让的最大值为8xRTT，修正假死问题。
_update 20160712: recompiled with go version go1.7rc1 darwin/amd64_
MD5 (kcptun-linux-amd64-20160701.tar.gz) = 3c55407a1c413677454c8620b384e71d
MD5 (kcptun-linux-386-20160701.tar.gz) = 395c33fca9d7d1f9474c7af47567221f
MD5 (kcptun-darwin-amd64-20160701.tar.gz) = d44508e3dc46ce82ab3cdabdf10ba3d8
MD5 (kcptun-darwin-386-20160701.tar.gz) = f21236c12e6d11aa03cc84f62abf5e81
MD5 (kcptun-windows-amd64-20160701.tar.gz) = 425d0b63e82bad5c84683c4fb3a35edf
MD5 (kcptun-windows-386-20160701.tar.gz) = 0972e5e3d637cd9ced8038e77adb9580
MD5 (kcptun-freebsd-amd64-20160701.tar.gz) = 524c0b1f1500e6ac81e38adc6422acb1
MD5 (kcptun-freebsd-386-20160701.tar.gz) = 74110c946da7dcf668720bc1e66e9a3c
MD5 (kcptun-linux-arm-20160701.tar.gz) = 00ce4b3a8dd4ff6c7e93db41172fa236