journalprocessor: Fix deserialize_message raising EOFError on the last message of each assignment

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).