Stream EC parts directly if/when possible

[roman-khimov]

Is your feature request related to a problem? Please describe.

I'm always frustrated when I realize we collect all relevant EC parts on intermediary node, concatenate them together and only after that we send the reply to user. This adds delay and processing overhead.

Describe the solution you'd like

Stream each part as it goes. If everything is available that's a clear win.

[roman-khimov]

Also, subsequent parts can use RANGE requests instead of GET so save on header handling.