remove "aliased class pool" caching approach

remove "aliased class pool" caching approach

Modified the ``JoinedLoader`` implementation to use a simpler approach in
one particular area where it previously used a cached structure that would
be shared among threads. The rationale is to avoid a potential race
condition which is suspected of being the cause of a particular crash
that's been reported multiple times. The cached structure in question is
still ultimately "cached" via the compiled SQL cache, so a performance
degradation is not anticipated.

The change also modifies the tests for None in context.secondary
to ensure no None values are in this list, as this is suspected
as being the immediate cause of the issue in #9777.  The cached
AliasedClass thing is suspected as being the origination of the
cause, as under high concurrency many threads might all access
that AliasedClass immediately, which seems a reasonable place
that the "adapter returning None" symptom might be originating.

As of yet we don't have a self-contained reproducer for the
issue, some initial attempts with threads are not showing any
issue.

Fixes: #9777
Change-Id: I967588f280796c413e629b55b8d97d40c1164248