db: Don't rely on branched connections

We were previously calling 'connect()' on the 'connectable' object in
'run_migrations_online', regardless of whether it was an 'Engine' or
'Connection' object. This worked because, as noted in an inline comment,
"when connectable is already a Connection object, calling 'connect()'
gives us a *branched connection*." This is no longer the case. From the
SQLAlchemy docs [1]:

  The Connection object does not support "branching", which was a
  pattern by which a sub "connection" would be used that refers to this
  connection as a parent.

Update our code to reflect this change, using the newly updated example
from the SQLAlchemy cookbook doc [2] as inspiration.

[1] https://docs.sqlalchemy.org/en/14/core/future.html#sqlalchemy.future.Connection
[2] https://alembic.sqlalchemy.org/en/latest/cookbook.html#connection-sharing

Closes-Bug: #2061375
Co-Authored-By: Stephen Finucane <stephenfin@redhat.com>
Change-Id: I5617dc7e6e97e4ba5aad2f5d8be40b2241b9cf4c

taskflow/persistence/backends/sqlalchemy/alembic/env.py

@@ -68,7 +68,12 @@ def run_migrations_online():
                               target_metadata=target_metadata)
             with context.begin_transaction():
                 context.run_migrations()
-
+    else:
+        context.configure(
+            connection=connectable,
+            target_metadata=target_metadata)
+        with context.begin_transaction():
+            context.run_migrations()
 
 if context.is_offline_mode():
     run_migrations_offline()