API changed, use event name as command name instead of event type in actor.wait.

The old i/f can't support register some type of event for more than twice. But
in some situations, we need wait two or more fd or timeout events. In another
hand, we must be careful that an *event name* can't register twice for all the
actors.

So, for the new interface, there are two things changed:

1. After we register the event with an *event name*, in actor.wait, we just need
   to wait the same *event name* instead of *fd_event* or *timeout*.
2. Each actor has its own namespace for event names, You only need to make sure
   that there is no same event name in actor itself.

Thanks Liu Peng(@van9ogh)'s idea.

Signed-off-by: Xiongfei Guo <xfguo@credosemi.com>