Review nginx/php-fpm process model

[calind]

1. Create a single pool for php
2. Reserve some % of that pool for admin requests
3. Reserve an extra 1-2 processes for php-fpm ping
4. Use https://github.com/openresty/lua-resty-limit-traffic for limiting and queueing connections to the reserverd pools.

This is needed to better handle liveness/readyness probes for the container. Related to bitpoke/wordpress-operator#57

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[stale]

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