Freeze models for healing migration

Take a snapshot of all models from the code base at the time when the
healing migration merges. The healing migration needs this frozen view
of the models to be available (even as the models change in the future)
to compare with the current DB schema. The healing migration will use
this comparison to heal the schema.

partially implement bp: db-migration-refactor

Change-Id: I438147c7961b1ecad47f6146cc7fc9396aee7bc5
ML2: Bind ports outside transactions

The ML2 plugin now calls the bind_port() operation on the registered
mechanism drivers outside of any enclosing DB transaction. Ports are
created or updated in one transaction, then a binding is established
if possible, and finally a second transaction commits the binding
result.

With [re]binding moved outside the DB transaction that triggered it,
it is now possible that multiple threads or processes will
concurrently try to bind the same port, or that the port will be
updated between transactions. Concurrent attempts to bind the same
port are allowed to proceed, which results are used is resolved in the
second transaction, and binding is retried if necessary.

Improvements to the Cisco Nexus driver and unit tests from Rich Curran
needed due to the binding changes are also included.

Closes-Bug: 1276391
Closes-Bug: 1335226
Change-Id: I65dafc330d6e812dad0667d2383858504d0ba299
Merge "ML2: Bind ports outside transactions"
Merge "Freeze models for healing migration"