QUIC: separated path validation retransmit backoff. Path validation packets containing PATH_CHALLENGE frames are sent separately from regular frame queue, because of the need to use a decicated path and pad the packets. The packets are sent periodically, separately from the regular probe/lost detection mechanism. A path validation packet is resent up to 3 times, each time after PTO expiration, with increasing per-path PTO backoff.

# Copyright (C) Igor Sysoev
# Copyright (C) Nginx, Inc.


have=NGX_SUPPRESS_WARN . auto/have

have=NGX_SMP . auto/have