[Maas-devel] Running with serializable isolation.

[Gavin Panella]

The first WIP proposal [1] builds up a retry helper for serialization
failures and integrates it into the `transactional` decorator.

The second WIP proposal [2] switches everything to SERIALIZABLE.

Both pass all tests run by bin/test.maas. I suggest using one of those
as a basis for further work.

They're both down the pipe from [3], so ATOMIC_REQUESTS is enabled and
TransactionMiddleware has been removed. This also means that they'll
actually fail in the Real World. You'll probably want to disable
ATOMIC_REQUESTS if you merge or branch either of these; see
fix_up_databases().

Have fun!

Gavin.


[1] https://code.launchpad.net/~allenap/maas/serializable-preparations/+merge/238992

[2] https://code.launchpad.net/~allenap/maas/serializable-to-the-moon/+merge/238993

[3] https://code.launchpad.net/~allenap/maas/transactions-redux-the-revenge/+merge/238864