[DBAL-54] Cannot drop schema using IDENTITY strategy in PostgreSQL

DBAL-54 was fixed in commit 3d55dc8eec235c09d64b061456295a1951299496 by
changing SQL queries order. However it was later reintroduced.

This patch uses a different, more future-proof way, approach by using
DROP SEQUENCE ... CASCADE syntax, and thus does not depend on queries order.
Merge pull request #289 from PowerKiKi/DBAL-54-bis

[DBAL-54] [DBAL-420] Cannot drop schema using IDENTITY strategy in PostgreSQL