bug#48118: 27.1; 28; Only first process receives output with multiple running processes

[Daniel Mendler]

When running multiple asynchronous processes only the output of the
first process is handled. This happens when the first process
continously produces a huge amount of output, for example when running
`ripgrep` as done by my `consult-ripgrep` command (part of my Consult
package). Then Emacs is stuck handling the output of the first process.
The output of the second process is not read until the first process is
terminated. I expect Emacs to treat the running processes fairly. The
issue also occurs if a :filter function is specified. Both Emacs 27 and
28 are affected.

Minimal reproducible example by @jakanakaevangeli:

(progn
  (setq pa (make-process
             :name "yes-a"
             :command '("yes")
             :connection-type 'pipe
             :buffer (setq a (generate-new-buffer " *a*"))))
  (setq pb (make-process
             :name "yes-b"
             :command '("yes")
             :connection-type 'pipe
             :buffer (setq b (generate-new-buffer " *b*"))))
  (run-at-time
   1 1
   (lambda ()
     (message "size a: %s\nsize b: %s"
              (buffer-size a) (buffer-size b)))))

Original bug discussions:
https://github.com/minad/consult/issues/272
https://github.com/minad/consult/pull/297