MEDIUM: server: implement check for dynamic servers

MEDIUM: server: implement check for dynamic servers

Implement check support for dynamic servers. The "check" keyword is now
enabled for dynamic servers. If used, the server check is initialized
and the check task started in the "add server" CLI handler. The check is
explicitely disabled and must be manually activated via "enable health"
CLI handler.

The dynamic server refcount is incremented if a check is configured. On
"delete server" handler, the check is purged, which decrements the
refcount.