JdbcIO should explicitly setAutoCommit to false

Hello,

Per PostgreSQL JDBC documentation, autocommit must be explicitly disabled on the connection to allow cursor streaming.

jkff mentionned it on the mailing list, however even if there is:

poolableConnectionFactory.setDefaultAutoCommit(false);

in JdbcIO:1555, currently, at least with JDBC driver 42.2.16, any read with JdbcIO will memoize the whole dataset (which leads to OOM), since

connection.getAutoCommit()

returns true in JdbcIO#ReadFn#processElement.

I can provide a PR — the patch is pretty simple (and solves the problem for us in 2.28.0):

if (connection == null) {
        connection = dataSource.getConnection();
}
connection.setAutoCommit(false); // line added

Thanks!