[core] bring client bindings up to date for configurable field names.

...va/com/yahoo/ycsb/db/cloudspanner/CloudSpannerClient.java

.../test/java/com/yahoo/ycsb/db/ElasticsearchClientTest.java

...oo/ycsb/db/elasticsearch5/ElasticsearchIntegTestBase.java

kudu/src/main/java/com/yahoo/ycsb/db/KuduYCSBClient.java

...est/java/com/yahoo/ycsb/db/rocksdb/RocksDBClientTest.java

.../test/java/com/yahoo/ycsb/db/solr/SolrClientBaseTest.java

...test/java/com/yahoo/ycsb/db/solr6/SolrClientBaseTest.java