IMPALA-675: Add function to get current default database

This uses the same syntax as postgres: current_database()

Change-Id: Ic6ca8ce1fe8c10a496800c45c58b0df4d214b51c
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1274
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins