Adding VARCHAR front & backend.

VARCHAR is treated as StringVal in the backend. All UDAs and UDFs which accept STRING
will also accept VARCHAR(N).

TODO: Reverted Avro codegen to fix Jenkins; needs separate patch.

Change-Id: Ifc120b6f0fe1f996b11a48b134d339ad3719331e
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/2527
Reviewed-by: Victor Bittorf <victor.bittorf@cloudera.com>
Tested-by: jenkins
(cherry picked from commit 3fcbf4f677b8e26c37eded4d8bb628e6fc53c1e9)
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4058