IMPALA-1013 GetFunctions is excessively long

In MetadataOp.getDbsMetadata(), it should only return function metadata
if function name is specified. It should not fetch any table metadata.

Change-Id: Ie0242b2cfd232d353b078a20ef65adda68d0e5d1
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2834
Tested-by: jenkins
Reviewed-by: Nong Li <nong@cloudera.com>