Connection & streams errors are now handled as prescribed by the
rfc.
Comment some implementation choices.
C-style prints switched to C++ streams.

Also correct a mistake in getting the target address: next_target() was
called at each udp send.

The double free was caused by a misuse of an unique_ptr which freed a
buffer owned by quicly. Replacing it by a vector make it work nicely.

uvw::UDPHandle::send takes ownership of the data only when it is passed
using an unique_ptr.

The connection is also properly freed, wich plugs a memleak.

This allows to send the connection close message as quickly as possible,
and potentially save some packet round trips.  E.g., on one particular
test instance, a 1-query burst goes from 23 udp packets exchanged to
only 12.

A segfault occured at quic connection close, when _quic_session was
reset, but packets where still comming.
An assert fail occured when receive_data was called on a close()'d quic
session.
Some extraneous prints removed.

With an adequately defined call to bind(), the handle knows if it must send
over IPv4 or IPv6.

The session ending of DoQ and of the tcp-based mode is slightly
different, putting them in the same function resulted in obscure code.

Main change is the use of length field, in the same way as DoT or DNS
over TCP.