Allow Output Buffer Flushing with PHP-FPM

As a systems administrator, I would like the mod_proxy flushpacket parameter added as a default configuration so that PHP-FPM can flush the output buffer and increase TTFB for large pages, instead of doing so manually.

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Apache 2.4.31 added support for mod_proxy's flushpackets parameter to mod_proxy_fcgi: http://archive.apache.org/dist/httpd/CHANGES_2.4.32

This parameter is required for PHP-FPM to successfully flush the output buffer. This can lead to substantial increases in TTFB for large pages, or when the output buffer is manually flushed.

I've currently implemented this in local template overrides by adding the following line inside the [%- IF vhost.php_fpm %] block:

<Proxy "fcgi://[% wildcard_safe(vhost.servername) %]" flushpackets=on></Proxy>

I've had good results with this on a server with 150+ domains on PHP-FPM so far, so would like to suggest this be implemented as a default configuration. This functionality should already be present when using any other PHP handler, so this is really only bringing PHP-FPM up to parity with other handlers.