get_apt_pkg_cache is slowing update-manager down

[Nathan Teodosio]

I noticed a long time for Update Manager to "complete", that means, showing the available update list.

Profiling shows that the private function "get_apt_pkg_cache" from ubuntu-pro-client:/usr/lib/python3/dist-packages/uaclient/apt.py is called multiple times, summing up to 17 s delay.

--->
% python3 -m cProfile -s tottime ./update-manager
WARNING:root:can not import unity GI Namespace Dbusmenu not available
         9479342 function calls (9456814 primitive calls) in 35.130 seconds

   Ordered by: internal time

   ncalls tottime percall cumtime percall filename:lineno(function)
       48 17.232 0.359 18.049 0.376 apt.py:287(get_apt_pkg_cache)
        1 4.795 4.795 34.356 34.356 Gtk.py:1690(main)
        3 1.192 0.397 1.250 0.417 cache.py:203(open)
   417580 0.822 0.000 2.629 0.000 cache.py:308(__iter__)
        3 0.799 0.266 0.802 0.267 {built-in method time.sleep}
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