locking: Make some aliased methods public

Some methods, such as “_is_owned” and “list_owned”, have been aliased to
make them public for a while now. This patch makes the actual
implementation public.

SharedLock's “is_owned” needs to be aliased to “_is_owned” to remain
compatible with Python's built-in threaded.Condition class.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>