OFConnectionManager: improve log messages

Reviewer: @kenchiang

This change attempts to improve the wording of log messages and add more
useful information. I also tweaked the log level of a couple of messages.

I added an assertion for the "inconsistent message length" case because it 
means there's a bug in the switch.
minor: it would be nice to have a macro for "of_object_id_str[magic_number]".