Reduce the size of the main Solr binary download

There has been a lot of recent discussion about how large the Solr download is, and how to reduce its size. The last release (4.10.2) weighs in at 143MB for the tar and 149MB for the zip.

Most users do not need the full download. They may never need contrib features, or they may only need one or two, with DIH being the most likely choice. They could likely get by with a download that's less than 40 MB.

Our primary competition has a 29MB zip download for the release that's current right now, and not too long ago, that was about 20MB. I didn't look very deep, but any additional features that might be available for download were not immediately apparent on their website. I'm sure they exist, but I would guess that most users never need those features, so most users never even see them.

Solr, by contrast, has everything included ... a "kitchen sink" approach. Once you get past the long download time and fire up the example, you're presented with configs that include features you're likely to never use.

Although this offers maximum flexibility, I think it also serves to cause confusion in a new user.

A much better option would be to create a core download that includes only a minimum set of features, probably just the war, the example servlet container, and an example config that only uses the functionality present in the war. We can create additional downloads that offer additional functionality and configs ... DIH would be a very small addon that would likely be downloaded frequently.

SOLR-5103 describes a plugin infrastructure which would make it very easy to offer a small core download and then let the user download additional functionality using scripts or the UI.