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:

gino/src/gino/api.py

Lines 213 to 227 in 8c888bd

	class GinoSchemaVisitor:
	__slots__ = ("_item",)
	def __init__(self, item):
	self._item = item
	def __getattr__(self, item):
	sync_func = getattr(self._item, item)
	async def _wrapper(bind=None, *args, **kwargs):
	if bind is None:
	bind = _bind_or_error(self._item)
	return await bind.run_sync(sync_func, *args, **kwargs)
	return _wrapper