Update from 1.15.6 to 1.15.7.
Update clojure and njs third-party modules to their recent versions.

MFH: 2018Q4

<ChangeLog>

*) Feature: the "proxy_requests" directive in the stream module.

*) Feature: the "delay" parameter of the "limit_req" directive.

Thanks to Vladislav Shabanov and Peter Shchuchkin.

*) Bugfix: memory leak on errors during reconfiguration.

*) Bugfix: in the $upstream_response_time, $upstream_connect_time, and

$upstream_header_time variables.

*) Bugfix: a segmentation fault might occur in a worker process if the

ngx_http_mp4_module was used on 32-bit platforms.

</ChangeLog>