Rewrite verifyIntegrity() to work with less memory

[Smoerrebroed]

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Feature request

Which Nextcloud Version are you currently using: (see administration page)
30.0.4

Is your feature request related to a problem? Please describe.
Yes. With < 256 M memory limit, verifyIntegrity() seems to fail silently with the current version.

Describe the solution you'd like
Make verifyIntegrity() less dependent on available memory. Right now, if the downloaded .zip file does not fit within the constraints of memory, it seems to silently fail which really throws off users without too much technical background. There is no mention of the root cause for the termination.

Describe alternatives you've considered
An alternative could be either skipping verification if the archive is too large (and alerting the user) or at least providing a config option to skip this verification.

Additional context
In some environments, it is not easily possible to increase the memory limit. Also, it is really hard to find out the root cause for this strange behaviour without any hint what is going (which is the current state of affairs).