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

	classGinoSchemaVisitor:
	__slots__= ("_item",)
	def__init__(self, item):
	self._item=item
	def__getattr__(self, item):
	sync_func=getattr(self._item, item)
	asyncdef_wrapper(bind=None, *args, **kwargs):
	ifbindisNone:
	bind=_bind_or_error(self._item)
	returnawaitbind.run_sync(sync_func, *args, **kwargs)
	return_wrapper