There was common situations where two provider instances where running
in parallel. And was creating weird errors (like getting wrong api_uri)
because the bootstrap deferreds were global but the Provider objects
not.

I don't like much singletons, but I think now is simpler than before.

- Resolves: #9073

Relates to #8918

(add some extra line to force tests to run again)

here we port the if-modified-since conditional mechanism, so that we
only write the config if it is newer than whan we have.

we also add a line with the status code to the logs, so that it's easier
to debug.

note that the 'configs.json' file is never returning 304.

- Resolves: #8773

node-mail chokes if we don't.