Fix memory usage regression in StatementWrapper iterator

Problem/Motivation

In #3174662: Encapsulate \PDOStatement instead of extending from it a PDO statement wrapper was introduced. The getIterator() method represents a performance regression relative to earlier versions of Drupal 8/9.

Previously if you did a foreach ($statement as $row) {} the PDO statement would fetch a single result from the database in each loop keeping memory consumption to a minimum.

Now, the getIterator() method fetches all results and constructs an ArrayIterator. For a large query site, this is large use of PHP memory.

Steps to reproduce

Compare memory usage before/after this change.

Proposed resolution

See, the API changes.

Remaining tasks

Add patch and tests.

User interface changes

n/a

API changes

• Introduce a Drupal\Core\Database\StatementWrapperIterator implementing \Iterator instead of \IteratorAggregate so that iteration on the statement object is kept in sync with the underlying client statement resultset cursor (net of the usage of PDO::MYSQL_ATTR_USE_BUFFERED_QUERY which is a PDO-only feature, other drivers do not necessarily have that, see #45 and #47).
• Let core MySql and PostgreSql database drivers implement the new class.
• Deprecate the class Drupal\Core\Database\StatementWrapper for removal in D11.

Data model changes

n/a

Release notes snippet