Allow COMPUTE STATS after changing the Avro schema in the schema file

The current way of handling Avro schema evolution to work with COMPUTE STATS without a need for refresh/invalidate is documented in IMPALA-4248. The request in that Jira is to make the order of the statements interchangeable, without the need for refresh.

This jira tracks a request to remove the need for a manual alter table after schema evolution altogether. This would mean that updating the .avsc file on HDFS (for tables defined via avro.schema.url), by itself, should be enough for Impala to pick up the new schema.