Full varnish cache invalidation is prevented by hardcoded backlog=200

[avoinea]

Is there any good reason we have Queue maxsize hard-coded to 200?!

plone.cachepurging/plone/cachepurging/purger.py

Line 69 in 3e714aa

def __init__(self, factory=Connection, timeout=30, backlog=200,

plone.cachepurging/plone/cachepurging/purger.py

Line 169 in 3e714aa

self.queues[key] = queue_ = queue.Queue(self.backlog)

This limit is easily reached if you have a lot of related-items to invalidate, thus Varnish cache will be partially invalidated resulting in inconsistent content.

I think this should be set by default to 0 (infinity)