Support aenter/aexit

[steersbob]

Defect

The following code would fail to await the aiohttp.ClientSession.close() coroutine, or attempt to close the session after the event loop closed.

with AsyncInfluxDBClient(mode='async') as client:
    pass

Desired behavior

def blocking_func():
    with AsyncInfluxDBClient(mode='blocking') as client:
        pass

async def async_func():
    async with AsyncInfluxDBClient(mode='async') as client:
        pass

Changes

• Added __aenter__, __aexit__, and close functions
• Removed __del__ function. For well-handled cleanup, clients can either explicitly close, or use the context guard. This also avoids unwanted side effects where __del__ is called after the event loop closes.
• async_client fixture is now async, and uses a module scoped event loop

[gusutabopb]

Look quite solid!

Thank you!