Add/Remove watchers at any time

[shafreeck]

We can just add/remove watchers before calling cetcd_multi_watch* so far. It is not flexible enough. What I expect is add or remove watchers at any time including after starting watching.

[shafreeck]

It is difficult to add/remove watchers dynamic after starting the multi_watch routine because curl is not thread-safety and it is complex and not expected to introduce a lock. I made it work by a more simple and reasonable way.

• cetcd_multi_watch/cetcd_multi_watch_async pass an array of watchers as an argument.
• cetcd_multi_watch/cetcd_multi_watch_async can be called multiple times with another array of watchers.

Now, with above mechanisms you can

• Add / Remove watchers before calling cetcd_multi_watch*
• Add more watchers and call cetcd_multi_watch* again
• Stop any watcher after calling cetcd_multi_watch*, it will be freed after being stopped.

See c0e6c5b