sql: rename changes() to row_count()

changes() is legacy function which came from SQLite. However, its name
is not perfect since it's "number of rows affected", not"number of rows
changed". Moreover, to make it real row count, we are nullifying counter
representing it in case operation is not accounted for DML or DDL.
For example, before this patch behavior was like:

INSERT INTO t VALUES (1), (2), (3);
START TRANSACTION;
SELECT changes();
---
- - [3]
...

As one can see, row counter remained unchanged from last INSERT
operation. However, now START TRANSACTION set it to 0 and as a
consequence row_count() also returns 0.

Closes #2181