feat: add combined size pool

[wayfarer3130]

Context

This PR lays the groundwork for adding display fetch controls that delay initial fetching of thumbnails and prefetches until the primary viewports are completed.

Changes & Results

Adds a prefetch queue for metadata
Adds an overall maximum size prefetch queue that applies to the metadata through prefetch queues, with the higher priority queues being done first. The intent of this is in an application such as OHIF to allow all the metadata queries to be sent off initially, and then let those continue to come in, while the initial images are not fetched initially. Then, the viewport specific images would be fetched and then thumbnails and finally prefetch images.

Testing

Link into OHIF, and display a study with thumbnails and prefetch. Add a log message on prefetch queue.
Reduce the delay in study browser to 125 ms or something like that, and all the thumbnails should be before the prefetch.
Should also add the metadata prefetch to the queue at level metadata, and add a viewport fetch for first image at low priority as soon as the display sets are created. That will demonstrate slowly fetching viewport/thumbnail images mixed in with metadata requests.

Checklist

PR

• [] My Pull Request title is descriptive, accurate and follows the
  semantic-release format and guidelines.

Code

• [] My code has been well-documented (function documentation, inline comments,
  etc.)

• [] I have run the yarn build:update-api to update the API documentation, and have
  committed the changes to this PR. (Read more here https://www.cornerstonejs.org/docs/contribute/update-api)

Public Documentation Updates

• [] The documentation page has been updated as necessary for any public API
  additions or removals.

Tested Environment

• [] "OS:
• [] "Node version:
• [] "Browser:

[netlify]

✅ Deploy Preview for cornerstone-3d-docs ready!

Name	Link
🔨 Latest commit	ac139f9
🔍 Latest deploy log	https://app.netlify.com/sites/cornerstone-3d-docs/deploys/67bc9dd806923e000815507d
😎 Deploy Preview	https://deploy-preview-1848--cornerstone-3d-docs.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.