BUG/MEDIUM: checks: Use the mux protocol specified on the server line

BUG/MEDIUM: checks: Use the mux protocol specified on the server line

First, when a server health check is initialized, it inherits the mux protocol
from the server if it is not already specified. Because there is no option to
specify the mux protocol for the checks, it is always inherited from the server
for now.

Then, if the connect rule is configured to use the server options, the mux
protocol of the check is used, if defined. Of course, if a mux protocol is
already defined for the connect rule, it is used in priority. But for now, it is
not possible.

Thus, if a server is configured to use, for instance, the h2 protocol, it is
possible to do the same for the health-checks.

No backport needed.