vconn: Distinguish between parse errors and other messages in rate-limiting.

vconn: Distinguish between parse errors and other messages in rate-limiting.

The vconn code wants to rate-limit errors, which there's not too much
point in reporting a lot of, from the log of all OpenFlow messages, which
are very important if we really want to log them at all.  So use a
different rate-limiter for each category.