Previously, only one client packet could be processed in a udp stream session
even though multiple response packets were supported.  Now multiple packets
coming from the same client address and port are delivered to the same stream
session.

If it's required to maintain a single stream of data, nginx should be
configured in a way that all packets from a client are delivered to the same
worker.  On Linux and DragonFly BSD the "reuseport" parameter should be
specified for this.  Other systems do not currently provide appropriate
mechanisms.  For these systems a single stream of udp packets is only
guaranteed in single-worker configurations.

The proxy_response directive now specifies how many packets are expected in
response to a single client packet.

The value is no longer guaranteed to be based on milliseconds
elapsed since Epoch.

If a socket was re-opened due to an error (02c2352d5b01 and fa0e093b64d7),
additional cleanup handler was installed each time.

FreeBSD returns EINVAL when getsockopt(TCP_FASTOPEN) is called on a unix
domain socket, resulting in "getsockopt(TCP_FASTOPEN) ... failed" messages
during binary upgrade when unix domain listen sockets are present in
the configuration.  Added EINVAL to the list of ignored error codes.

Previously, only unix domain sockets were reopened to tolerate cases when
local syslog server was restarted.  It makes sense to treat other cases
(for example, local IP address changes) similarly.