Bufferevent triggering and watermark introspection

When juggling data callbacks like in [1] or [2], one might need a way to fire them, subject to watermarks and the bufferevent's deferred/unlocked status.

[1] https://gist.github.com/mistotebe/6252608
[2] https://github.com/mistotebe/skeeter/blob/WIP/chaining.c#L79