Stage 1: Added optimistic concurrency control to _update function

[Fikavec]

optimisticConcurrencyMode - int, indicates optimistic concurrency strategy from Solr Docs includeVersionField - bool, tell Solr to return version field after add/update document

Details in https://solr.apache.org/guide/6_6/updating-parts-of-documents.html#UpdatingPartsofDocuments-OptimisticConcurrency

#367