[SPARK-2590][SQL] Added option to handle incremental collection, disabled by default

JIRA issue: [SPARK-2590](https://issues.apache.org/jira/browse/SPARK-2590)
OK. I put it in `SQLConf` because of exactly the same reason [Zongheng mentioned](https://github.com/apache/spark/pull/1819/files#r15913270), and also had mixed feelings about this. I'd prefer to have something like Hive `ConfVars` to define all configurations, but let's do it later.