Remove PDU queuing

Remove PDU queuing

Tweak ideated during the commit message of the previous commit.

- If the read() yields at least one Error Report, drop the connection.

This is because all the server-received error codes currently defined
are supposed to result in immediate connection termination.

If a future RFC defines a nonfatal error code, Error Reports should
probably be downgraded to the 'last PDU' rule below.

- Otherwise, if a read() yields multile PDUs, drop all except for the
  last one.

Since it's what the client is most likely expecting, I guess. Serial
Queries and Reset Queries are alternate means to achieve the same goal,
so it doesn't make sense to queue them.