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: Ia90d7d6d3ed590dbe2cfd981139c24e512dc5df2
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/2527
Reviewed-by: Victor Bittorf <victor.bittorf@cloudera.com>
Tested-by: jenkins