Gracefully handle forced shutdown for Publish background tasks

[suzmue]

If the background task that contains the Publish actors gets forcefully stopped (not from dropping the Publisher), the oneshot channels that it holds don't get correctly resolved and we get the unexpected error when polling the channel.

We can resolve this in two ways:

1. Handle the closed channel on the PublishHandle side
2. impl Drop for the BatchActors and resolve all PublishHandle's it knows about

1 will likely be easier to implement.

[suzmue]

We should be able to recreate this in a test by

1. keeping a handle to the background task spawned for the Publisher
2. sending some messages (that shouldn't complete a batch)
3. aborting the spawned task
4. awaiting on the handles

We do not currently hold on to that handle when the Publisher is built.