R1:cae59d83456e

Add an option to make it easier to debug page hangs

Summary:
Fixes T6044. We've had two cases (both the same install, coincidentally) where pages got hung doing too much data fetching.

When pages hang, we don't get a useful stack trace out of them, since nginx, php-fpm, or PHP eventually terminates things in a non-useful way without any diagnostic information.

The second time (the recent Macros issue) I was able to walk the install through removing limits on nginx, php-fpm, php, and eventually getting a…