Reduce FEC memory usage
- Reduce FEC memory usage(>30%), limited FEC queue to 3 * (dataShard+parityShard) packets.
$ go version
go version go1.4.2 linux/amd64
xtaci/smux@b4a6fb8
xtaci/kcp-go@2a9a3f7
xtaci/kcptun@229a4a8
$ go version
go version go1.4.2 linux/amd64
xtaci/smux@b4a6fb8
xtaci/kcp-go@2a9a3f7
xtaci/kcptun@229a4a8