MINOR: check: implement check-pool-conn-name srv keyword

MINOR: check: implement check-pool-conn-name srv keyword

This commit is a direct follow-up of the previous one. It defines a new
server keyword check-pool-conn-name. It is used as the default value for
the name parameter of idle connection hash generation.

Its behavior is similar to server keyword pool-conn-name, but reserved
for checks reuse. If check-pool-conn-name is set, it is used in priority
to match a connection for reuse. If unset, a fallback is performed on
check-sni.