Never create a channel-log doc for channel "*"

There would be too much contention because every write updates it.

This patch actually fixes a bug because "*" was only half-implemented:
the channel log would be written the first time the _changes feed asked
for "*", but it would never get updated on doc writes, so it would keep
returning the same stale data forever.
Now the "*" channel is always fetched from the view and never written
to a log. This is slow, but that's OK because this channel will only
rarely be used, probably for admin purposes.