bug#43406: Emacs 27.1 memory consumption grows indefinitely

[Ludovic Courtès]

More precisely:

--8<---------------cut here---------------start------------->8---
- timer-event-handler                                     805,671,418  87%
 - apply                                                  804,987,874  87%
  - helm-ff--cache-mode-refresh                           802,103,698  86%
   - helm-ff--cache-mode-refresh-1                        801,321,290  86%
    - helm-ff-refresh-cache                               801,237,818  86%
     - maphash                                            801,237,818  86%
      + #<compiled 0xec7961>                              801,237,818  86%
      helm-ff--cache-mode-max-idle-time                         6,384   0%
   + run-with-idle-timer                                      651,464   0%
--8<---------------cut here---------------end--------------->8---