MINOR: quic: replace usage of ssl_sock_ctx by quic_conn

MINOR: quic: replace usage of ssl_sock_ctx by quic_conn

Big refactoring on xprt-quic. A lot of functions were using the
ssl_sock_ctx as argument to only access the related quic_conn. All these
arguments are replaced by a quic_conn parameter.

As a convention, the quic_conn instance is always the first parameter of
these functions.

This commit is part of the rearchitecture of xprt-quic layers and the
separation between xprt and connection instances.