MEDIUM: check: Revamp the way the protocol and xprt are determined

MEDIUM: check: Revamp the way the protocol and xprt are determined

Storing the protocol directly into the check was not a good idea,
because the protocol may not be determined until after a DNS resolution
on the server, and may even change at runtime, if the DNS changes.
What we can, however, figure out at start up, is the net_addr_type,
which will contain all that we need to find out which protocol to use
later.
Also revert the changes made by commit 07edaed1918a6433126b4d4d61b7f7b0e9324b30
that would not reuse the server xprt if a different alpn is set for
checks. The alpn is just a string, and should not influence the choice
of the xprt.
We'll now make sure to use the server xprt, unless an address is
provided, in which case we'll use whatever xprt matches that address, or
a port, in which case we'll assume we want TCP, and use check_ssl to
know whetver we want the SSL xprt or not.
Now that the check contains all that is needed to know which protocol to
look up, always just use that when creating a new check connection if it
is the default check connection, and for now, always use TCP when a
tcp-check or http-check connect rule is used (which means those can't be
used for QUIC so far).

This should hopefully fix github issue #3324.