remove deprecated -f/--forks option

Problems with --forks feature:

• Does not support dynamic restart (related: #268 (closed))
• Does not support watchdog
• First process is single point of failure
• Per-instance configuration via environment variables is harder
• Fixing this practically means re-implementing systemd or supervisord, which is obviously a bad idea.

Related: #529 (closed)

Task list:

• remove -f option and related forking code
• worker.count should also be removed
• remove -f usage from all testing scripts, deckard, respdiff etc.
• update our benchmakring docker image to be able to run multiple kresd instances without -f

changed milestone to %6.0.0

changed title from remove deprecated -f option to remove deprecated -f/--forks option

changed the description

mentioned in commit 7414de27

mentioned in commit 6460a003

mentioned in commit 822e4342

changed the description

mentioned in commit 8c35bb85

mentioned in merge request !1602 (merged)

marked the checklist item remove -f option and related forking code as completed

marked the checklist item worker.count should also be removed as completed

mentioned in commit a72780a0

closed with merge request !1602 (merged)

mentioned in commit cabbfd18