Collection Aliasing.

We should bring back the old aliasing feature, but for SolrCloud and with the ability to alias one collection to many.

The old alias feature was of more limited use and had some problems, so we dropped it, but I think we can do this in a more useful way with SolrCloud, and at a level were it's not invasive to the CoreContainer.

Initially, the search side will allowing mapping a single alias to multiple collections, but the index side will only support mapping a single alias to a single collection.