Watcher vs Watching in storage pkg is confusing. Watcher should be named
as watchStream since it contains a channel as stream to send out events.
Then we can rename watching to watcher, which actually watches on a key
and send watched events through watchStream.
This commits renames watcher to watchStram.
One watcher includes multiple watchings, and their events are
sent out through one channel. For the received event, user would like to
know which watching it belongs to.
Introduce a watch ID. When watching on some key, user will get a watch
ID. The watch ID is attached to all events that is observed by this
watch.
