manager: cache-clear command via HTTP API

• The command to clear the cache is on /cache-clear HTTP API endpoint.
• Body for the HTTP request is defined by CacheClearRPCSchema.
• Lua command cache.clear() is created using jinja template.
• The command is sent to one already running worker using it's unix-socket. kresd_controller/registered_workers.py
• cache-clear command added to kresctl + documentation
• Documentation for the cache clear operation.

Implements #876 (closed)