OFConnectionManager: improve log messages

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.
Merge into master from pull request #161:
OFConnectionManager: improve log messages (https://github.com/floodlight/indigo/pull/161)