Add allow_sync_call=False

[dkang-quora]

This parameter allows the synchronous calls in asyncio mode. It's used to migrate the existing synchronous function to @asynq() in an incremental way.

@asynq(allow_sync_call=False)
def fn(..): ...

with AsyncioMode():
    fn()

Test:

>>> import asynq
>>> asynq.asynq_to_async._asyncio_mode.set(True)
<Token var=<ContextVar name='asyncio_mode' default=False at 0x7f7170e1de90> at 0x7f7171966d40>
>>> @asynq.asynq(allow_sync_call=True)
... def fn(): ...
... 
>>> fn()
asyncio mode does not support synchronous calls: <function fn at 0x7f7170da6d40> at <stdin>