R1:81a7c9ac43f2

Remove the execution time limit (if any) before sinking HTTP responses

Summary:
Ref T12907. At least part of the problem is that we can hit PHP's `max_execution_time` limit on the file download pathway.

We don't currently set this to anything in the application itself, but PHP often sets it to 30s by default (and we have it set to 30s in production).

When writing responses, remove this limit. This limit is mostly a protection against accidental loops/recurison/etc., not a process slot protection. It…