[SPARK-17229][SQL] PostgresDialect shouldn't widen float and short types during reads

## What changes were proposed in this pull request?

When reading float4 and smallint columns from PostgreSQL, Spark's `PostgresDialect` widens these types to Decimal and Integer rather than using the narrower Float and Short types. According to https://www.postgresql.org/docs/7.1/static/datatype.html#DATATYPE-TABLE, Postgres maps the `smallint` type to a signed two-byte integer and the `real` / `float4` types to single precision floating point numbers.

This patch fixes this by adding more special-cases to `getCatalystType`, similar to what was done for the Derby JDBC dialect. I also fixed a similar problem in the write path which causes Spark to create integer columns in Postgres for what should have been ShortType columns.

## How was this patch tested?

New test cases in `PostgresIntegrationSuite` (which I ran manually because Jenkins can't run it right now).

Author: Josh Rosen <joshrosen@databricks.com>

Closes #14796 from JoshRosen/postgres-jdbc-type-fixes.