complete upload - skip db access for small object (for performance)

[alphaprinz]

Explain the changes

For small objects with one part, we know the start/end of object.
There's no need to fix values in db at the end of the put object.
(It's needed for multipart upload where each part doesn't know its start/end at single part upload).
Also, set part as committed when inserting the part (again, to skip db access for removing uncommitted at the end).

Issues: Fixed #xxx / Gap #xxx

Redundant db access at the end of upload.

Testing Instructions:

• Doc added/updated
• Tests added

[guymguym]

I would prefer to move this optimization into _complete_object_parts() (need to send params.num_parts inside).

Over there we can start by identifying a single part case, and optimize the hell of it, so that we don't even read the part from the db, which is an aggressive optimization which we also need to explain why it is safe (the reason is that mpu cannot have a single part per multipart... which is a bit of a "bad" assumption to take because it could easily get broken one day by someone who just doesn't expect it calling these apis).

We can also add a simpler condition that will still read the parts, but will skip updating them by noticing that they are already committed and do not require an offset/seq update - simply surround the push to parts_updates with a condition like

if (part.uncommitted || updates.set_updates) {
    context.parts_updates.push(updates);
}

on this line -

noobaa-core/src/server/object_services/object_server.js

Line 2099 in cdd266f

context.parts_updates.push(updates);

[alphaprinz]

I've moved the check whether to skip the db into _complete_next_parts().
Since we're going for performance, I figured it's best to skip the db network hop altogether.
You think it's not safe enough?

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