Implemented RETURNING support for SQL

[egormanga]

Description of Changes

SQL RETURNING for DML to return affected rows to the caller:

INSERT INTO test (a, b)
VALUES (1, 2)
RETURNING *;

gives:

 a | b 
---+---
 1 | 2 

(same goes for UPDATE & DELETE).

Individual columns can be specified, too — separately from the fields the statement operates on:

UPDATE test
SET a = 1
WHERE b < 2
RETURNING b;

allowing to efficiently combine DML with implicit SELECTs.

API and ABI breaking changes

None.

Expected complexity level and risk

2.

Testing

• Manual testing
• Unit tests

[bfops]

Thank you for filing this! We'll work on getting a reviewer on it.