JS API should only request logs it hasn't seen yet

[niloc132]

The JS API internally plays back old logs, and the server will also keep around old logs (to a point) to replay for new connections or reconnects. We should pick one or the other so that we provide the sorts of guarantees about usage that we would like.

Another approach could be to offer a Table instance which holds the logs, so that it follows the same API as every other table.