Push down filters for SHOW PARTITIONS

We need to pass a Constraint to Metadata.getLayouts() in two places:

• ShowQueriesRewrite
• InformationSchemaPageSourceProvider

This allows the connector to filter for a query like this:

SHOW PARTITIONS FROM orders WHERE orderdate = '2016-02-03'

[electrum]

Hive partition columns can be of any type, not just strings.

[wenleix]

In ShowQueriesRewrite.visitShowPartitions, what we really need are just partition columns. Thus instead of calling metadata.getLayouts, we can add another method getDiscrtePredicateColumns for metadata.

For the execution of rewriting query, we will change the schema of __internal_partitions__ to have fixed number partition columns (e.g. 10). So we are not going to support more than 10 partition columns in Hive, but it allows predicate pushdown to avoid fetch all the partitions.