Rename JournalClient.max_messages to JournalClient.stop_after_objects

After various refactorings, the meaning of max_messages got muddled and we're
at a point where it doesn't mean anything anymore. stop_after_objects is
clearer as to what behavior the parameter is actually trying to achieve. We also
rename the replay command-line argument to the same name.

These refactorings also had us end up with a loop inside JournalClient.process,
while some callers still had a loop around it which ever got called once (or 0
times when the surrounding code was buggy). This commit removes all these outer
loops as well, keeping only the JournalClient.process inner loop.

While we're here, we use the opportunity to clarify and expand the documentation
of the JournalClient.