Model observer to message-passing and index upgrade

[luis100]

Change the current methodology of indexing from a model observer pattern to a message-passing pattern using Akka, which may allow distribution into micro-services (having index actor in another container).

Allow use case for Solr major upgrade, where information is indexed in both Solr 7 and Solr 8 but Solr 7 is used as a service dependency, a re-index all is issued and allows Solr 8 to replace Solr 7.

The same strategy could then be used to incorporate different systems that must listen to all model changes, like migrating from Solr to ElasticSearch, or having a GraphDB for some of the information when metadata is in RDF.