Avoid regenerating JdbcSchema for every table in a query

Currently CalcitePlanner generates a complete JdbcSchema for every JdbcTable in the query.

https://github.com/apache/hive/blob/master/ql/src/java/org/apache/hadoop/hive/ql/parse/CalcitePlanner.java#L3174

This wastes some resources since every call to JdbcSchema#getTable needs to communicate with the database to bring back the tables belonging to the schema. Moreover, the fact that a schema is created during planning is counter-intuitive since in principle the schema shouldn't change.