There's nothing useful they can do with it anyway.

…acklog

Will now report -1 any time a message comes directly from a specific channel. It is not possible for an arbitrary backend implementation to efficiently provide the `global_id` of messages fetched from channel-specific storage mechanisms.

Also probably improves performance by fetching full backlogs less often.

Subscribing from some future point is unnecessary and heavy to implement. See ae0d7cd#commitcomment-31541983

When client code tries to subscribe to a channel using a `last_id` that doesn't exist on that channel, we raise an exception.

See ae0d7cd#commitcomment-31544627 for details.