Decide on a mail delivery upgrade workflow

MX will now deliver messages using the Soledad Incoming API to write pending blobs that the user will download and process. We have to decide how this will be enabled by the platform.

Kali has made a proposal that goes like this:

Kali Kaneko @kali commented a month ago

So, the proposed solution for the upgrade is:

• We upgrade the code with the optional incoming-blob switch in mx.
• The packages shipped with platform continue to use the legacy mx-couchdb delivery.
• If the sysadmin adds the neded info (soledad-incoming token) AND incoming endpoint is available, the delivery switches to the incoming endpoint.
• it's up to the soledad configuration to decide if the mail delivered to the incoming endpoint is written to couch or goes to the blobs namespace.
• we need to have e2e test to show that incoming-couch combination work.
• at some point in the future, the upgrade process (ie, the debian package) could take care of either generating the token both in soledad and mx, or ask for an unattended upgrade (ie, deprecation of the direct couchdb writes from mx).

I really would like to have this mx ability to manually switch to incoming-endpoint shipped before platform 0.10. @shyba do you think it's possible?