use getsockopt to check connection state after poll indicate writable

[MOON-CLJ]

No description provided.

[mckelvin]

Could you add more details on why we need this? It'd would best if you could add a test case or a piece of POC code to reproduce the issue you want to fix.

[MOON-CLJ]

@mckelvin this pr contains two points，

1，as suggestion by https://man.openbsd.org/connect.2 ，also see the https://man.openbsd.org/connect.2#EXAMPLES 。 add getsockopt with option SO_ERROR after poll indicate ok to double check after connect。it's not easy to add a test。
2，again，add more log with connection's pointer address to expose connection's state。

[tclh123]

not necessary?

[MOON-CLJ]

@tclh123 https://github.com/douban/libmc/blob/master/include/Common.h#L61 log will use errno，if not clean here。such as EINPROGRESS EALREADY above will show in the log，it's noisy。