Fix self-referential many-to-one query

Add tests for one-to-many self-referential relation
VersionExpressionReflector now replaces column with a value
only when the column is in the local side of the relation
Many-to-one subquery now uses an alias; previously, the query
would crash on self-referential relations
Fix many-to-many self-referential query

Now correctly use the relation's secondaryjoin to form the subquery,
replacing custom logic that was erroneous on self-referential cases.
Add tests for many-to-many self-referential
rename variable
Merge branch 'self-referential' of https://github.com/dtheodor/sqlalchemy-continuum into dtheodor-self-referential
Merge branch 'dtheodor-self-referential'