[SPARK-16674][SQL] Avoid per-record type dispatch in JDBC when reading

## What changes were proposed in this pull request?

Currently, `JDBCRDD.compute` is doing type dispatch for each row to read appropriate values.
It might not have to be done like this because the schema is already kept in `JDBCRDD`.

So, appropriate converters can be created first according to the schema, and then apply them to each row.

## How was this patch tested?

Existing tests should cover this.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #14313 from HyukjinKwon/SPARK-16674.