Hide solr path under internal_solr

*Mostly copied from #312*

The Solr update handler (`solr/core/update`) is well known to any Solr user. Old Riak Search supported indexing via this same interface. It would allow a user to create index entries without a KV object. Yokozuna has no such interface as I'm not sure what the semantics would be. E.g. would it simply index things in solr or would it generate KV objects too?

The problem is that the local Solr nodes that Yokozuna starts do have the update handler in place and they are listening on the canonical URL. This might confuse users, thinking that they can send writes to this URL and it will distributed the data and work with Yokozuna when it will in fact not do any such thing.

Renaming base `/solr` url to `/internal_solr` to make it more difficult for users to accidentally connect. That wouldn't stop users from making this mistake but it would make it a little harder to shoot yourself in the foot.