Fix some instance tracking to prevent Python-side objects from leaking
buffers.
Also, reduce min size of Cache instance (real minimum should be around ~~19MiB,
but we need to make it more deterministic and 20 MiB seems to be reasonable).
Still some stuff left to do, but it needs more investigation and, for
now, this should suffice just to enable some form of CI.
Signed-off-by: Jan Musial <jan.musial@intel.com>
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