Smooth dataflow curve & Half CPU usage for client

(Strongly Recommended for upgrading)

1. Smooth dataflow curve by setting MaxStreamWindowSize to sockbuf(hidden).
2. Half CPU usage by using recv() syscall instead of recvfrom() syscall. (client only)

（强烈建议升级）

1. 设置MaxStreamWindowSize 值为 sockbuf(隐)，以期望平滑数据传输曲线。
2. 调整UDP接收的系统调用，recvfrom() 改为 recv()，降低客户端CPU消耗(一半以上)。

Limit max RTO backoff to 8xRTO

Limit max RTO backoff to 8xRTT.
限定RTO退让的最大值为8xRTT，修正假死问题。

Compression with Snappy (The first compression enabled release)

CAUTION: NOT COMPATIBLE WITH PREVIOUS RELEASES WITH DEFAULT SETTINGS.

MAKE SURE TO VERIFY YOUR MD5 HASH BEFORE DEPLOY.
STRONGLY RECOMMENDED FOR UPGRADING.

Add google snappy compression for transport layer to accelerate data transfer(enabled by default) , use -nocomp on both side to disable.

中文:
注意：默认与以前版本不兼容

部署之前请务必确认您的MD5哈希值
强烈建议升级

使用google snappy算法对传输层做数据流压缩（默认开启），用于加速数据传输，在两端同时使用 -nocomp 禁止压缩传输。

support multiple connections for client

(Update 0616: for everyone who is using 0615 version, should upgrade to this one)
(Update 0616: compile with go 1.7beta2, much faster)
Multiple physical connection supported(try -conn 4), slightly alleviated the problem 'Head Of Line' Blocking
https://en.wikipedia.org/wiki/Head-of-line_blocking
QUIC solved this problem by stream based ARQ, not connection based.