support dynamic add peer to compatible with dyups module

[yzprofile]

[xiaokai-wang]

Dynamic_add/del is not compatible with nginx-1.8.0-, is that ok to be merged into master?

[iandyh]

This will be very useful. Thanks a lot.

cc @yaoweibin

[IYism]

@yaoweibin it is indeed what I need.

[wanjidong]

@yaoweibin Thank you very much, both of these are very much needed ~