BUG/MINOR: check: only try connection reuse for http-check rulesets

BUG/MINOR: check: only try connection reuse for http-check rulesets

In 3.2, a new server keyword "check-reuse-pool" has been introduced. It
allows to reuse a connection for a new check, instead of always
initializing a new one. This is only performed if the check does not
rely on specific connection parameters differing from the server.

This patch further restricts reuse for checks only when an HTTP ruleset
is used at the backend level. Indeed, reusing a connection outside of
HTTP is an undefined behavior. The impact of this bug is unknown and
depends on the proxy/server configuration. In the case of an HTTP
backend with non-HTTP checks, check-reuse-pool would probably cause a
drop in reuse rate.

Along this change, implement a new diagnostic warning on servers to
report that check-reuse-pool cannot apply due to an incompatible check
type.

This must be backported up to 3.2.