reduce cpu-load in endless loop

When RequestedPublishingInterval isn't provided, calculating the RequestedMaxKeepAliveCount raises a DivisionByZero exception.

Therefore let's calculate RequestedMaxKeepAliveCount with a conservative PublishInterval value when this occurs.

Worth noting our server doesn't revise the PublishInterval value, hence will not start the publication with a value of 0.
https://github.com/FreeOpcUa/opcua-asyncio/blob/master/asyncua/server/internal_subscription.py#L50-L51

Fixes #490

* refactor recursive to iterative approach of awaiting cancelled tasks

* close all tasks instead of one and rename loop method.
Co-authored-by: Randolph Busch <randolph.busch@ticos.ch>