Flymake: speed up mode-line display with simple cache

Flymake: speed up mode-line display with simple cache

When scrolling heavily (perhaps holding down C-n), the function
'flymake-mode-line-counter' showed very prominently in the memory and
CPU profiles.

Using a cache here was trivial, if not particularly pretty.  The
function all but disappears from the profiles afterwards.

* lisp/progmodes/flymake.el (flymake--mode-line-counter-cache):
New variable.
(flymake--publish-diagnostics): Flush cache aggressively.
(flymake-mode-line-error-counter): Adjust.
(flymake--mode-line-counter-1): New helper from earlier
flymake--mode-line-counter.
(flymake--mode-line-counter): Use cache.