The db field value version check, which is a preflight to
major upgrades (to detect if a prior upgrade was not completed)
was using model_query, which could orphan an open transaction in
the same process until the python interpretter went and took out
the perverable trash.
We now use an explicit session which structurally ensures we close
any open transactions which allows a metadata lock to be obtained
to perform a schema update..
Change-Id: Id51419bc50af5a756bb7b0ca451df1936dd6f904
93688e9531