MINOR: quic: Initialize the connection timer asap

MINOR: quic: Initialize the connection timer asap

We must move this initialization from xprt_start() callback, which
comes too late (after handshake completion for 1RTT session). This timer must be
usable as soon as we have packets to send/receive. Let's initialize it after
the TLS context is initialized in qc_conn_alloc_ssl_ctx(). This latter function
initializes I/O handler task (quic_conn_io_cb) to send/receive packets.