feat: support leastconn" strategy

[JayLZhou]

solved issue.#205
Although I added a file to rewrite net.conn, it seems a bit complicated, but in fact it only adds a layer of judgment to the leastconn strategy, and there is no change in other aspects. In order to realize the transition from user mode to kernel mode, I have to do it
I did some implementations to demonstrate
I opened three agents here 127.0.0.1:8080 127.0.0.1:1080 127.0.0.1:8081

2020/12/11 16:27:45 server.go:162: [http] 127.0.0.1:56405 <-> www.baidu.com:80 via 127.0.0.1:8080
2020/12/11 16:27:45 server.go:162: [http] 127.0.0.1:56408 <-> www.alibaba.com:80 via 127.0.0.1:1080
2020/12/11 16:27:45 server.go:162: [http] 127.0.0.1:56413 <-> www.tencent.com:80 via 127.0.0.1:8081
2020/12/11 16:27:48 server.go:162: [http] 127.0.0.1:56422 <-> www.baidu.com:80 via 127.0.0.1:8080
2020/12/11 16:27:48 server.go:162: [http] 127.0.0.1:56425 <-> www.alibaba.com:80 via 127.0.0.1:1080
2020/12/11 16:27:48 server.go:162: [http] 127.0.0.1:56428 <-> www.tencent.com:80 via 127.0.0.1:8081
I can do more tests if you need

[phantomcraft]

I'm waiting for @nadoo for merging in the dev branch to test it.

[gaby]

@JayLZhou Probably worth updating the README too.

@nadoo Any chance of getting this?