Changed default get_message timeout behaviour to block execution instead of poll indefinately with 100% CPU usage

[NadavTasher]

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Description of change

As can be seen here, when using get_message (Pub/Sub) without specifying a timeout, the default behaviour is to not block but instead return immediately, causing infinite loops running at 100% CPU.
With asyncio this is extremely frustrating since the whole point of await is to wait for what is supposedly a blocking operation.

[lfos]

For visibility: This PR addresses issue #3208. I hope we can get this merged soon, as it's been causing me (and others) a fair amount of headache.

[NadavTasher]

I would love that!
Anything I can do?