Optimisation of JAR downloading

With the new cbng-webservice implemented in !49 (merged) (great job @pbudzyns!) we gain a separation between the "resolve" and "download" phase installation - this was previously not possible, and was one of the motivators for the change in the first place.

Now that we know what we need to download, and are in charge of the download itself, we have an opportunity to be more efficient when re-resolving an environment: we can re-use previously downloaded JARs!

Proposal: Rather than first clearing the JAR directory, and then downloading all of the JARs that we need (possibly re-downloading something we just cleared away), let's keep the JARs that we know are needed and only remove the ones that are not.