OFConnectionManager: convert write queue list to a ringbuffer

This gets rid of the linear list append and the extra allocation/free.
OFConnectionManager: write as many messages as possible to the socket

The previous limit of 32 messages per tick was creating a bottleneck. I think
that in producer-consumer scenarios like this we should give the consumer
priority and not limit how much it can consume. Otherwise the work done by the
producer will be wasted when the message is dropped.
OFConnectionManager: improve comments

Also fixed an incorrect function prototype.
Merge into master from pull request #145:
Convert OFConnectionManager write queue to a ringbuffer (https://github.com/floodlight/indigo/pull/145)