continuous execution should sequence committed epochs

Currently, the EpochCoordinator doesn't enforce a commit order. If a message for epoch n gets lost in the ether, and epoch n + 1 happens to be ready for commit earlier, epoch n + 1 will be committed.

This is either incorrect or needlessly confusing, because it's not safe to start from the end offset of epoch n + 1 until epoch n is committed. EpochCoordinator should enforce this sequencing.

Note that this is not actually a problem right now, because the commit messages go through the same RPC channel from the same place. But we shouldn't implicitly bake this assumption in.