WatchListDeletable does not properly implement Waitable

[shawkins]

The current expectation, and even after the proposed changes for #3271, a list is not waitable - only a single resource.

This can be addressed by changing

public interface WatchListDeletable<T, L> extends VersionWatchAndWaitable<T>

to

public interface WatchListDeletable<T, L> extends Watchable<Watcher<T>>, Versionable<WatchAndWaitable<T>>

Or I can generalize the logic in #3274 to support waiting on a list.