version conflict on updating

[nvdk]

in mu-search 0.9.0-beta.6 version conflict errors have been reported.

When upserting the following error is returned by elasticsearch

{"error":{"root_cause":[{"type":"version_conflict_engine_exception","reason":"[http://themis.vlaanderen.be/id/besluitvormingsaangelegenheid/E14FB551-3347-11ED-B8A0-F82C0F9DE1CF]: version conflict, required seqNo [160], primary term [1]. current document has seqNo [161] and primary term [1]","index_uuid":"lxSwyE2sRKO514gxWawDEg","shard":"0","index":"16f4ac90cf8b3ea359636d37ade2b542"}],"type":"version_conflict_engine_exception","reason":"[http://themis.vlaanderen.be/id/besluitvormingsaangelegenheid/E14FB551-3347-11ED-B8A0-F82C0F9DE1CF]: version conflict, required seqNo [160], primary term [1]. current document has seqNo [161] and primary term [1]","index_uuid":"lxSwyE2sRKO514gxWawDEg","shard":"0","index":"16f4ac90cf8b3ea359636d37ade2b542"},"status":409}

This indicates the document has been updated while we're upserting. mu-search should ensure we only try to update the document once.