Stack SolrRequestInfo

Sometimes SolrRequestInfo need to be suspended/overridden with a new one that is used temporarily. Examples are in the [subquery] transformer, and in warm of caches, and in QuerySenderListener (another type of warming), maybe others. This can be annoying to do correctly, and in at least one place it isn't done correctly. SolrRequestInfoSuspender shows some complexity. In this issue, dsmiley proposes using a stack internally to SolrRequestInfo that is push'ed and pop'ed. It's not the only way to solve this but it's one way.

See linked issues for the context and discussion.