Replace nested indexer with native SOLR indexing

[jeremyf]

Is your feature request related to a problem? Please describe.
In the 2.9.0 implementation of Hyrax the nested indexer is non-performant. In the pending 3.5 and 4.0 release there is work to move away from the non-performant indexer and leverage a native SOLR graph indexing.

Describe the solution you'd like
That work has been quasi-ported to the hyrax-v2_graph_indexer gem. It is worth exploring if this work could be brought further back to v2.9.0 by adjusting the implementation details of that graph indexer.

Assuming that 2.9.0 and 2.9.6 are adequately compatible in the low-level implementation details it should be straight forward to adjust the dependencies of the hyrax-v2_graph_indexer gem.

This would require further testing to ensure functional needs are still met. But the performance improvements should be substantive.