adjustments for unreliable gc

sporadic (and at the moment persistent) test failures
related to aiosqlite seem to have in common that Python
gc stops working fully when we run a lot of tests with
aiosqlite.  The failures are not limited to aiosqlite
as they are more involving places where we assume or
expect gc.collect() to get rid of things, and it doesn't.

Identify (based on reproducible case on the d3 CI runner)
the spots where this happens and add fixes.

test/orm/test_transaction.py test_gced_delete_on_rollback
has always been a very sensitive test with a lot of issues,
so here we move it to the test_memusage suite and limit
it only to when the memusage suite is running.

Change-Id: I683412d0effe8732c45980b40722e5bb63431177

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