OFConnectionManager: cleanup logging

This change makes the INFO loglevel more user-friendly.

- Redundant log messages are demoted to VERBOSE or TRACE.
- Connection ids are replaced with the human-readable address.
- Errno numbers are replaced with strerror.
- A few important logs are promoted to WARN or ERROR.

Fixes XE-40.
Merge into master from pull request #17:
OFConnectionManager: cleanup logging (https://github.com/floodlight/indigo/pull/17)