Reduce flicker in Isearch mode

Reduce flicker in Isearch mode

Lazy highlighting now happens immediately when the search string is at
least as long as the value of the new custom variable
`lazy-highlight-no-delay-length`.  Also avoid updating the lazy
count in the echo area too often.
* isearch.el (lazy-highlight-no-delay-length): New defcustom.
* isearch.el (lazy-lazy-count-format): Avoid a momentarily incorrect
count when reversing search direction.
* isearch.el (isearch-lazy-highlight-new-loop): Avoid a call to
`isearch-message` that is quickly succeed by a second echo area
update, thus causing flicker.
* isearch.el (isearch-lazy-highlight-new-loop):
Start lazy highlight immediately if appropriate.
* etc/NEWS: Announce the change.
* doc/emacs/search.texi: Document `lazy-highlight-no-delay-length'.

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