solrj CloudSolrClient: add directUpdatesToLeadersOnly support
(Marvin Justice, Christine Poerschke)
- Addition of a directUpdatesToLeadersOnly flag to allow clients to request that direct updates be sent to the shard leaders and only to the shard leaders.
- In a scenario where there is temporarily no shard leader the update request will 'fail fast' allowing the client to handle retry logic.
- SOLR-6312 concerns the ((currently) no longer used) updatesToLeaders flag. The updatesToLeaders logic however appears to be slightly different from the proposed directUpdatesToLeadersOnly logic: updatesToLeaders indicates that sending to leaders is preferred but not mandatory whereas directUpdatesToLeadersOnly mandates sending to leaders only.