MINOR: quic: define quic_cc_algo as const

MINOR: quic: define quic_cc_algo as const

Each QUIC congestion algorithm is defined as a structure with callbacks
in it. Every quic_conn has a member pointing to the configured
algorithm, inherited from the bind-conf keyword or to the default CUBIC
value.

Convert all these definitions to const. This ensures that there never
will be an accidental modification of a globally shared structure. This
also requires to mark quic_cc_algo field in bind_conf and quic_cc as
const.