Refactoring concurrency in scaler's pending HTTP queue fetcher logic

[arschles]

This patch refactors the queuePinger's requestCounts function in the scaler. Highlights:

• Renames requestCounts to fetchAndSaveCounts
• Creates a new fetchCounts function (not method on the queuePinger) that does all the heavy lifting
• Changes fetchAndSaveCounts to simply call fetchCounts and save the results internally to the queuePinger
• Adds tests for fetchCounts
• Ensures that all counts are fetched when fetchCounts returns
• The queue tick duration is now configurable

Checklist

• Commits are signed with Developer Certificate of Origin (DCO)
• Any necessary documentation is added, such as:
  • README.md
  • The docs/ directory
  • The docs repo
• Refactor requestCounts to update all counts before it returns
• Write tests that call requestCounts directly
• The queue pinger should not start the background goroutine in its constructor. A separate function call should be required for that.

Fixes #286
Fixes #139

[arschles]

cc/ @asw101

[bketelsen]

LGTM

[arschles]

Thanks @bketelsen !

Although the linkinator CI check is failing, I'm going to merge this anyway. The failure is due to kedacore/keda#2215, and the work to fix it will be done separately.