Synchronize async fixtures dynamically

[seifertm]

Asynchronous fixtures are currently synchronized as part of the pytest_pycollect_makeitem hook. This forces pytest-asyncio to provide a separate code path for fixtures. That involves different synchronization wrappers for coroutines and async generators as well as custom code for fixture rebinding.

To simplify the future maintenance of pytest-asyncio, fixtures should be synchronized dynamically when a test requests them.