Add ConnStore invariant that ConnHandler exists for VBConn

MB-41719 was an issue where tests segfaulted because a VBConn exists in the ConnStore but the corresponding ConnHandler (that shoud "own" it) does not exist. This issue only escaped attention of the test developers as the test ran in a multi-threaded test suite and would fail in a background thread running at the wrong time. Ideally this is a condition the ConnStore should catch, but adding it breaks a lot of tests as they break this invariant so this wasn't done at the time.