Don't break internal loops on CancelledError when the cancel is not triggered internally (avoid CTRL-C issue on python < 3.11)

[diorcety]

Fix for #287
The issue comes from handling of KeyboardInterrupt before 3.11 (python/cpython@f08a191)
Before 3.11 all the tasks are cancelled when a KeyboardInterrupt is generated (https://github.com/python/cpython/blob/3.10/Lib/asyncio/runners.py#L47), leading that all internal loops are also cancelled, for example the client reader, flusher loops. Any call to nats client after a CTRL-PY will certainly timeout

In order to avoid such issue, this commit change the behaviour of CancelError inside the loop if this one is not due to internal cancel call.
The call to unsubscribe on PushSubscription is changed. Subscription loop is looking for is_closed from Subscription object, calling unsubscribe on super object of PushSubscription doesn't set correctly the flag _closed on the underlying Subscription.

I found during this work that the subscription inside _init_request_sub is never unsubscribed correctly.