Improve engine selection

[CaselIT]

Currently when read_from_rw_engines or write_to_rw_engines are False and no engine is read-only or write-only all the engine are used.

falcon-sqla/falcon_sqla/manager.py

Lines 41 to 44 in b75af08

	def _filter_by_role(self, engines, role):
	filtered = tuple(engine for engine in engines
	if self._engines.get(engine) == role)
	return filtered or engines

This behavior is quite strange, since the option is not respected.

Raising an error in this case would probably be more appropriate, the problem with the exception is that by adding the engines one by one it's easy to encounter an error while adding them.

I'm not sure how to best support this scenario.
One option is to make the engine selection lazy, so that only on the first use we check for this, the user does not have an immediate feedback with this approach