MINOR: checks: Store the protocol to be used in struct check

MINOR: checks: Store the protocol to be used in struct check

When parsing the check address, store the associated proto too.
That way we can use the notation like quic4@address, and the right
protocol will be used. It is possible for checks to use a different
protocol than the server, ie we can have a QUIC server but want to run
TCP checks, so we can't just reuse whatever the server uses.
WIP: store the protocol in checks