src-impaladb: first commit of ImpalaDB support

  * includes modifications to support explict column scoping
    required to support joins with ImpalaDB
  * semi-joins are done through the explicit LEFT SEMI JOIN syntax
  * loading tables requires HDFS support via rhdfs
  * note: IMpapaDB does not support anti-joins
  * Force USE dbname as hive2 driver does not honour URI
  * Fix column alias
  * column alias is not always calculated correctly as it comes
    from a nested sub query
  * fix db_list_tables to scope to DB
  * ImpalaDB can't create temporary tables
  * add option to documentation