SNOW-2920022: SessionManager leaks memory due to defaultdict usage for sessions map

While developing https://github.com/Snowflake-Labs/django-snowflake and after fixing #2725, a Django test reveals that SessionManager.__init__() leaks memory due to defaultdict usage for the SessionManager._sessions_map:

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FAIL: test_select_related_memory_leak (select_related.tests.SelectRelatedTests.test_select_related_memory_leak)
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Traceback (most recent call last):
  File "/home/tim/code/django/tests/select_related/tests.py", line 149, in test_select_related_memory_leak
    self.assert_no_memory_leaks()
  File "/home/tim/code/django/tests/select_related/tests.py", line 73, in assert_no_memory_leaks
    self.assertEqual(gc.garbage, [])
AssertionError: Lists differ: [<snowflake.connector.session_manager.Sess[330 chars] {})] != []

First list contains 5 additional elements.
First extra element 0:
<snowflake.connector.session_manager.SessionManager object at 0x7bb6daff8dd0>

+ []
- [<snowflake.connector.session_manager.SessionManager object at 0x7bb6daff8dd0>,
-  <cell at 0x7bb6daffb6a0: SessionManager object at 0x7bb6daff8dd0>,
-  (<cell at 0x7bb6daffb6a0: SessionManager object at 0x7bb6daff8dd0>,),
-  <function SessionManager.__init__.<locals>.<lambda> at 0x7bb6dad16160>,
-  defaultdict(<function SessionManager.__init__.<locals>.<lambda> at 0x7bb6dad16160>, {})]

I'm not sure how to fix this offhand.