MEDIUM: server: Introduce the concept of path parameters

MEDIUM: server: Introduce the concept of path parameters

Add a new field in struct server, path parameters. It will contain
connection informations for the server that are not expected to change.
For now, just store the ALPN negociated with the server. Each time an
handhskae is done, we'll update it, even though it is not supposed to
change. This will be useful when trying to send early data, that way
we'll know which mux to use.
Each time the server goes down or is disabled, those informations are
erased, as we can't be sure those parameters will be the same once the
server will be back up.