IMPALA-1080: Implement "SET <query_option>" as SQL statement.

Also add support for "SET", which returns a table of query options and
their respective values.

The front-end parses the option into a (key, value) pair and then the
existing backend logic is used to set the option, or return the result
sets.

Change-Id: I40dbd98537e2a73bdd5b27d8b2575a2fe6f8295b
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3582
Reviewed-by: Daniel Hecht <dhecht@cloudera.com>
Tested-by: jenkins