This improves the remote watching mechanism as follows: Watch requests
are intercepted by the RemoteWatcher and not sent on the wire,
excepted watches from the remoteWatcher itself.
RemoteWatcher is then in charge of forwarding DeathWatchNotification
messages to the watchers.
This reduces the number of watch message to one per watchee, even if
there are several watcher on the same watchee (instead of n+1 before).
Reversed watch messages, and watch on ref with undefinedUid are excluded from
interception by the RemoteWatcher and so are handled as before this commit.
In addition, the following changes are made:
- Keep watchers in a map watchee -> watchers for more efficient retrieval
(in a scala Multimap)
- Keep watchees in a map address -> watchee for more efficient retrieval
(in a scala Multimap)
- Use of InternalActorRef more thoroughly to avoid casts
- Rewatch use a standard watch message, as the distinction is longer needed
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