switch to db tmp table when possible

[piccolbo]

blocked by https://issues.apache.org/jira/browse/SPARK-6339?jql=text%20~%20%22CREATE%20TEMPORARY%20TABLE%20as%20select%22%20AND%20project%20%3D%20Spark

attempts to clean up tmp tables from R in a robust way have not been very successful because the connection gets destroyed early.