Move ComponentBuildTrace creation from before_commit to before_flush

Since ComponentBuildTrace(s) get created with db_session.commit() call,
is is not possible to commit more items in bulk if they already have been flushed.

Current unit-tests' setup can be significantly sped up if items can be quickly
flushed on the fly and bulk-commited only once at the end. Moreover in general it
seems more appropriate/safer to handle this in before_flush as any implicit
or accidental flush could cause new build traces not to be created at all. As flush
is implicitly called before every commit anyway, this change shouldn't pose any harm.

module_build_service/common/models.py

@@ -1302,7 +1302,7 @@ class LogMessage(MBSBase):
         )
 
 
-def session_before_commit_handlers(session):
+def session_before_flush_handlers(session, flush_context, instances):
     # new and updated items
     for item in set(session.new) | set(session.dirty):
         # handlers for component builds