[SPARK-18947][SQL] SQLContext.tableNames should not call Catalog.listTables

## What changes were proposed in this pull request?

It's a huge waste to call `Catalog.listTables` in `SQLContext.tableNames`, which only need the table names, while `Catalog.listTables` will get the table metadata for each table name.

## How was this patch tested?

N/A

Author: Wenchen Fan <wenchen@databricks.com>

Closes #16352 from cloud-fan/minor.

(cherry picked from commit b7650f11)
Signed-off-by: Wenchen Fan <wenchen@databricks.com>