Aborting sync request

[pavelloz]

Research if its easy/moderately easy to do:

• User is syncing a.css which is 1MB
• During upload, user adds again a.css to queue
• Abort first request, because we know already that new version is in queue

This will be very helpful when working with tailwind (1+MB css files in dev mode), and extremely helpful on slow connections.

Tests with 50KBps upload rate generated a lot of 502 (even on concurrency=1).

[z-x]

Needs to be confirmed if sync uses Direct Upload