batches.py: Don't pass SQLAlchemy objects between threads

SQLAlchemy objects can't be used from multiple threads - so when starting
threads for builds, pass the ComponentBuild id rather than the object.
(Note that despite the comment that the threads were sharing a session,
they weren't - what was passed to the thread was a scoped_session that
acts as a separate thread-local session per-thread.)

BUILD_COMPONENT_DB_SESSION_LOCK - a threading.Lock() object that was used
in a few places - but not nearly enough places to effectively lock usage
of a shared session - is removed.

module_build_service/common/models.py

@@ -1107,6 +1107,10 @@ class ComponentBuild(MBSBase):
         Index("idx_component_builds_build_id_nvr", "module_id", "nvr", unique=True),
     )
 
+    @classmethod
+    def from_id(cls, db_session, component_build_id):
+        return db_session.query(cls).filter(cls.id == component_build_id).first()
+
     @classmethod
     def from_component_event(cls, db_session, task_id, module_id=None):
         _filter = db_session.query(cls).filter