Enable an external index (Solr) to receive a minimal index-update
document (Solr document) based on messaging emitted from an F4
repository

Implement customizable search index design (https://wiki.duraspace.org/display/FF/Design+-+Customizable+Search+Index)

Also, allow asynchronous or synchronous indexing patterns for both Solr
and SPARQL indexing.
Merge pull request #17 from futures/ImplMappingForSolrFields

Impl mapping for solr fields