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