Correct prefix counters

EffectiveRibInWriter's installedPrefixes counter is not accurate and cannot be accurately maintained during write process, because we do not know whether a prefix ended up being installed and hence when we see a withdraw we would have to read the data we have written.

Instead of current expensive maintenance, we should maintain these number solely on events we trigger as writes - i.e. we should subscribe to the EFFRIBIN table routes for a particular writer and observe the number of routes present in the datastore.

This way the counter-keeping will be dead-simple and also accurate.