Fixed #313, remove stack when empty

[fantix]

1. I found that it is not possible to use ctx.reset(token), because for now we allow releasing connections in different order, and possibly in different tasks:

async def task1():
    async with engine.acquire() as conn1:
        loop.create_task(task2())
        await asyncio.sleep(1)
    
async def task2():
    async with engine.acquire() as conn2:
        await asyncio.sleep(2)

asyncio.run(task1())

In this case, both tasks are sharing the same stack, but not reusing each other. The stack was created in task1, but cleared out in task2. ContextVar.reset(token) requires that the given token was created in the same context (task), therefore cannot be used in this PR.

2. There was another approach to create a new empty stack on acquire() if the current one is empty. Other than resource waste, there was another reason came to me not to do so, but as writing this PR I cannot remember the reason :sigh

[coveralls]

Pull Request Test Coverage Report for Build 1186

• 49 of 49 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 98.263%

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Totals
Change from base Build 1184:	0.01%
Covered Lines:	3961
Relevant Lines:	4031

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💛 - Coveralls

[wwwjfy]

I haven't tried, but I think using 2. in the description, while the first task is getting the connection, the second task may find the stack created by the first task is empty then remove it?

[wwwjfy]

👏

[fantix]

Yeah it was very much that. Thanks!