MTU selection starts by doubling the initial MTU until the first failure.
Then binary search is used to find the path MTU.

Previously, NGX_AGAIN returned by ngx_quic_send() was treated by
ngx_quic_frame_sendto() as error, which triggered errors in its callers.
However, a blocked socket is not an error.  Now NGX_AGAIN is passed as is to
the ngx_quic_frame_sendto() callers, which can safely ignore it.

The frames for which the padding is removed are PATH_CHALLENGE and
PATH_RESPONSE, which are sent separately by ngx_quic_frame_sendto().

Its value is the opposite of path->validated.

When probe timeout expired while congestion window was exhausted, probe PINGs
could not be sent.  As a result, lost packets could not be declared lost and
congestion window could not be freed for new packets.  This deadlock
continued until connection idle timeout expiration.

Now PINGs are sent separately from the frame queue without congestion control,
as specified by RFC 9002, Section 7:

  An endpoint MUST NOT send a packet if it would cause bytes_in_flight
  (see Appendix B.2) to be larger than the congestion window, unless the
  packet is sent on a PTO timer expiration (see Section 6.2) or when entering
  recovery (see Section 7.3.2).

Previously, PTO handler analyzed the first packet in the sent queue for the
timeout expiration.  However, the last sent packet should be analyzed instead.
An example is timeout calculation in ngx_quic_set_lost_timer().

Previously the field pnum of a potentially freed frame was accessed.  Now the
value is copied to a local variable.  The old behavior did not cause any
problems since the frame memory is not freed, but is moved to a free queue
instead.

In non-GSO mode, a datagram is sent if congestion window is not exceeded by the
time of send.  The window could be exceeded by a small amount after the send.
In GSO mode, congestion window was checked in a similar way, but for all
concatenated datagrams as a whole.  This could result in exceeding congestion
window by a lot.  Now congestion window is checked for every datagram in GSO
mode as well.