This caused `JournalClientOffsetRanges` to ignore the last batch of messages
in each assignment, because `JournalClient.handle_messages` deserializes
all messages in the batch before calling the worker function;
and raising `EOFError` from `deserialize_message` makes it exit early
(before calling the worker fn).

Additionally, it doesn't make much sense for a `deserialize_message`
function to raise this kind of exception.

Instead, this commit removes the explicit `raise EOFError`, and tells
`JournalClient` to stop on EOF. `deserialize_message` calls
`handle_offset`, which updates the assignment of the Kafka consumer to
be the empty set, which causes it to be EOF (since there are no more
partitions to read from).