Avoid flicker in lazy-highlight by doing all updates without redisplay.

Avoid flicker in lazy-highlight by doing all updates without redisplay.

* lisp/isearch.el (lazy-highlight-max-at-a-time):
Change default value from 20 to nil to not trigger redisplay
between updating iterations.
(lazy-highlight-cleanup): New arg ‘procrastinate’ to not remove
overlays when non-nil.
(isearch-lazy-highlight-new-loop): Call lazy-highlight-cleanup
with non-nil second arg when the search string is not empty.
Run timer with isearch-lazy-highlight-start instead of
isearch-lazy-highlight-update.
(isearch-lazy-highlight-start): New function. (Bug#25751)