Backfill Improvements, Hot Fix follow-up.

[AlfredoG87]

• 1.fix(backfill): Backfill Logic refactor to reduce serverStatus requests to the absolute necessary and on-demand edge case fix #1952 (comment), use Test Fixtures instead of mocks. Looked at this further, and for the specific case doing a test fixture is an overkill and be the same result.
• 2. fix(backfill): Backfill Logic refactor to reduce serverStatus requests to the absolute necessary and on-demand edge case fix #1952 (comment), Replace Mock with real Counters in Test Classes, same as above, the mock serves its purpose as intended, no need to change.
• 3. fix(backfill): Backfill Logic refactor to reduce serverStatus requests to the absolute necessary and on-demand edge case fix #1952 (comment), More Comments on Test Class path
• 4. fix(backfill): Backfill Logic refactor to reduce serverStatus requests to the absolute necessary and on-demand edge case fix #1952 (comment), Continguos ranges check and optimization.
• 5. fix(backfill): Backfill Logic refactor to reduce serverStatus requests to the absolute necessary and on-demand edge case fix #1952 (comment), Improve Getting Availability Logic
• 6. fix(backfill): Backfill Logic refactor to reduce serverStatus requests to the absolute necessary and on-demand edge case fix #1952 (comment), Missing UTs for Node Selection scenarios
• 7. fix(backfill): Backfill Logic refactor to reduce serverStatus requests to the absolute necessary and on-demand edge case fix #1952 (comment), Health Penalization Improvements