OFStateManager: don't send zero debug counters

We're now keeping hundreds of counters per connection, about half of which will
always be zero. The clients aren't interested in zero counters, and if they
were they could infer their existence from the desc stats reply.
Merge into master from pull request #108:
OFStateManager: don't send zero debug counters (https://github.com/floodlight/indigo/pull/108)