Homestyx hydra

R1:9875af739fb5

When we purge the request cache, also force PHP to collect cycles

Summary:
Ref T12997. See that task for more details. Briefly, an unusual dataset (where commits are mentioned hundreds of times by other commits) is causing some weird memory behavior in the daemons.

Forcing PHP to GC cycles explicitly after each task completes seems to help with this, by cleaning up some of the memory between tasks. A more thorough fix might be to untangle the `$xactions` structure, but that's significantly more involved.
Repository: R1 hydra
Commit Date: Sep 28 2017