MINOR: quic-be: add a "CC connection" backend TX buffer pool

MINOR: quic-be: add a "CC connection" backend TX buffer pool

A QUIC client must be able to close a connection sending Initial packets. But
QUIC client Initial packets must always be at least 1200 bytes long. To reduce
the memory use of TX buffers of a connection when in "closing" state, a pool
was dedicated for this purpose but with a too much reduced TX buffer size
(QUIC_MAX_CC_BUFSIZE).

This patch adds a "closing state connection" TX buffer pool with the same role
for QUIC backends.