CPU optimizations

1. CPU optimizations(including syscall, segment merging, frame allocation).
2. Wait maximum -interval for packet merging before sending.
3. Fix a possible CONNECTION STALL in smux.
4. Reduce default parameter -interval due to the improvements of code execution.

(Uploaded at March 22, 20:20, Everyone is suggested to upgrade to this version.)

$ go version
go version go1.8 darwin/amd64
acf7a01
xtaci/kcp-go@993d72d
xtaci/smux@a1a5df8

63a7a4bd5cb4075ee584225c2cff82eae1ae3e7d  kcptun-linux-amd64-20170322.tar.gz
130cb7229871ff06fb0ffda2051bf41bac5e72cc  kcptun-linux-386-20170322.tar.gz
25bd350a3fc6bd5dce566a220ab76da1d6f61ac6  kcptun-darwin-amd64-20170322.tar.gz
4933cd73416fb013e7f69806931f927ce00fb00e  kcptun-darwin-386-20170322.tar.gz
82095425eab1531cf6952390b5a0faa40ad6c40a  kcptun-windows-amd64-20170322.tar.gz
9d7e18ea690454f063db12d83837d533d3ef4f7c  kcptun-windows-386-20170322.tar.gz
c8c2b680b9c405559e47b7acb917ae0ece09e179  kcptun-freebsd-amd64-20170322.tar.gz
3cb3fb14601e57d2508f6d685dd1c879df2662f0  kcptun-freebsd-386-20170322.tar.gz
bde3196ed45764d85cddf685bb45f9e3b56536d7  kcptun-linux-arm-20170322.tar.gz
f90d15322ad12c043b790d3afd15745a335a2dc0  kcptun-linux-mipsle-20170322.tar.gz
560e75b983e05a3386f1be5b5036011dda582d3a  kcptun-linux-mips-20170322.tar.gz