Support for transactions with BEGIN ... END and/or multi-insert statements

[jvdwrf]

Currently, it's possible to create transactions with rollback, however these transactions have to communicate multiple times with the database.

It would be great if transactions like the following would be supported:

--! multi_table_insert(param_a, param_b):
BEGIN;

INSERT INTO a (param_a) VALUES (:param_a);
INSERT INTO b (param_b) VALUES (:param_b);

COMMIT;

Additionally, support for statements with multiple INSERTs:

--! some_function(..)
WITH value AS (
    INSERT INTO ... VALUES ... RETURNING ...
) INSERT INTO ... VALUES ...;