IMPALA-1104: Fixes for compute stats on Avro tables. Create Avro tables without col defs.

This patch addresses the following issues:
1. Allow creating Avro tables without col defs in Impala. Compute stats works on them.
2. Handle table creation with inconsistent col defs and Avro schema as follows:
   The table creation will succeed and ignore the col defs in favor of the Avro schema.
   A warning is issued that the col defs and the Avro schema are inconsistent.
   Compute stats works on such tables.

This patch does not address the issue of compute stats after Avro schema evolution.

Change-Id: Iea6b737d238d81491dc2097012ebc149a89d03ba
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4182
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: Alex Behm <alex.behm@cloudera.com>