New SolrJ CollectionScopedSolrClient

[dsmiley]

This SolrClient delegates to a backing client with a specified collection. Simple attempts to refer to another collection will fail. DISCLAIMER: this isn't a security mechanism.

There are tests and places in client code I've seen that would benefit from this. For one thing, it enables taking an existing SolrClient that doesn't have a defaultCollection (e.g. JettySolrRunner.getSolrClient()) and simply wrap it. Voila. I'm working on Jetty -> Apache transition and in the process I'm moving needless client creations to use existing clients, and I want to use this. I don't want this to "sneak in" within a test-focused PR.

I'll probably add a changelog but wasn't going to add a JIRA.

[dsmiley]

It could use a nice toString -- I'll add one: delegate + "/" + defaultSolrCollection

[dsmiley]

Out of scope here... I remain troubled by the excessive overloading of methods on SolrClient that take a collection param. I'd rather see them all gone; SolrClient just talks to a node but has a method to return a CollectionOperationsClient interface implementation (a new thing). It wouldn't subclass SolrClient; it's just a bunch of methods users expect send requests that are collection-scoped.