MEDIUM: quic: Send CONNECTION_CLOSE packets from a dedicated buffer.

MEDIUM: quic: Send CONNECTION_CLOSE packets from a dedicated buffer.

Add a new pool <pool_head_quic_cc_buf> for buffer used when building datagram
wich CONNECTION_CLOSE frames inside with QUIC_MIN_CC_PKTSIZE(128) as minimum
size.
Add ->cc_buf_area to quic_conn struct to store such buffers.
Add ->cc_dgram_len to store the size of the "connection close" datagrams
and ->cc_buf a buffer struct to be used with ->cc_buf_area as ->area member
value.
Implement qc_get_txb() to be called in place of qc_txb_alloc() to allocate
a struct "quic_cc_buf" buffer when the connection needs an immediate close
or a buffer struct if not.
Modify qc_prep_hptks() and qc_prep_app_pkts() to allow them to use such
"quic_cc_buf" buffer when an immediate close is required.