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"