Limit concurrent uploads to one

[PVince81]

Description

Limit concurrent uploads to one to avoid clogging the network queue of the browser.
The progress bar is still added before the upload actually starts, but the SDK upload method is called only once the p-queue triggers it when it's its turn.

Related Issue

Fixes #2594

Motivation and Context

Unclog the browser network queue

How Has This Been Tested?

• Test case 1: upload many files in regular files view and observe the network console:
  • before the fix: a lot of pending entries, laptop fan going crazy
  • after the fix: only one pending upload network call at a time
• Test case 2: like test case 1 but using the "upload-only" public link share (aka files-drop) because this has another code path

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised: