Improved pool and factory

[Fabrice Bacchella]

I changed the way factory are identified. Instead of using a single default, factory can be using different URI and filter which one they are interested in. It allows to have many instance of the same kind that will filter the URI. For example using mongo:instance1:path and mongo:instance2:path.

With this I also removed the limitation on the pool that it can uses only subclass of RrdFileBackendFactory, it can use any that is put in the active list, see
http://rrd4j.org/site/apidocs/org/rrd4j/core/RrdBackendFactory.html#addFactories(org.rrd4j.core.RrdBackendFactory...)
and
http://rrd4j.org/site/apidocs/org/rrd4j/core/RrdBackendFactory.html#findFactory(java.net.URI)

Please feel free to try and comment it.