Attempt to be compatible with synchronous event callbacks

[wwwjfy]

based on #702, refs #691, a hacky fix

Honestly, Gino doesn't have to support table creation/deletion, as this can go farther, like requests of async version of Alembic, though it can be done in another package.

[fantix]

Right - the schema.py will be completely removed in GINO 1.4, replaced by a very simple bridge:

https://github.com/python-gino/gino/blob/8c888bd6abe0e877602d55aff33d2f67c9cbcd03/src/gino/api.py#L213-L227