Create pluggable indexing queue with pluggable callbackhandlers to add extra custom indexing logic

Currently, the solr-client has a SolrManagerImpl that does crawl the repository during start up and index all beans into solr. However, this shouldn't be the responsibility of the SolrManagerImpl.

The crawler should be pluggable, and delegate what needs to indexed to some executor queue. The actual indexing of a contentbean should also get a callbackhandler (containing the SolrDocument, ContentBean, etc) method that developer can implement to append extra custom indexing logic