Efficient attachment downloading for mailbox export

[charlag]

We should optimize attachment data downloading for mailbox export.

Naive approach of downloading each blob one by one leads to N+2 requests.

There are two possible optimizations we can apply:

• For attachments in our own archive utilize archive-scoped token. This gets rid of per-blob token requests.
• For attachments in other archives utilize a token that works for multiple instances.
• Download multiple blobs of the same attachment together if the size permits it
• Download multiple blobs of different attachments if they belong to the same archive and the size permits it

Context

Downloading blobs requires requesting a blob access token first. The tokens can be scoped to individual instances or to the whole archives.

Archives are associated with a group. Sent attachments and externally-received attachments will belong to the mail group archives, while confidentially-received attachments do belong to the archive associated with the sender.

At the moment when downloading raw blobs (like for attachments) client does request tokens for each instance.

Estimated: 24h
Time Taken: 7h + 4h + 5h