Update from 1.16.0 to 1.17.0.

This release introduces two handy features, please take a look
on http://mailman.nginx.org/pipermail/unit/2020-April/000199.html
for details.

<ChangeLog>

*) Feature: a "return" action with optional "location" for immediate

responses and external redirection.

*) Feature: fractional weights support for upstream servers.

*) Bugfix: accidental 502 "Bad Gateway" errors might have occurred in

applications under high load.

*) Bugfix: memory leak in the router; the bug had appeared in 1.13.0.

*) Bugfix: segmentation fault might have occurred in the router process

when reaching open connections limit.

*) Bugfix: "close() failed (9: Bad file descriptor)" alerts might have

appeared in the log while processing large request bodies; the bug
had appeared in 1.16.0.

*) Bugfix: existing application processes didn't reopen the log file.

*) Bugfix: incompatibility with some Node.js applications.

*) Bugfix: broken build on DragonFly BSD; the bug had appeared in

1.16.0.

</ChangeLog>