[SNAP-2158] DataFrame update API.

[rishitesh]

Changes proposed in this pull request

Added update API to DataFrame. Both row and column tables can be updated df.write.update("table")

Also added a utility method to get JDBC data frames using pooled connection settings. Otherwise, JDBC relation will create a new connection every time, which is very expensive in streaming applications.

Added a check in SnappyHashJoin to skip scanning of tables, if the built map size is zero. This is helpful if some join operation is happening in an empty dataframe.

Patch testing

pre-checkin

ReleaseNotes.txt changes

Na

Other PRs

NA