Memory consumption improvements

In DataPool, following can be done to reduce memory usage:

• ArtifactDescriptor key does not have to be Artifact instance, is really just GACEV needed (is identity) WRONG, it must be Artifact. Still, some improvement is done introducing new key type.
• ArtifactDescriptors could have "interned" the List<Dependency> fields as well

Latter is done and controlled by two new configuration properties:

• aether.dependencyCollector.pool.internArtifactDescriptorDependencies defaults to false
• aether.dependencyCollector.pool.internArtifactDescriptorManagedDependencies defaults to true

Having true/true tremendously improve memory consumption but increases runtime.