SolrClient implementations should not mutate request objects

Currently, our "load balancing" SolrClient implementations mutate the SolrRequest instances they're operating on, changing the "basePath" as a way to dictate which Solr node the request is ultimately routed to.

This is effective for the LB client's purposes, but it can cause problems especially in SolrJ code that reuses SolrRequest objects for multiple requests. (This is pretty common - Solr itself and its test code does this frequently).

This ticket aims to cover restructuring our "LB" client code so that it no longer quietly mutates the objects provided by callers.