Instant Search: set a timeout for URL rewriting

#### Steps to reproduce the issue

- Enter “posts” as your search query.

#### What I expected

Press back to go to the previous page.

#### What happened instead

Every character entered in the search box triggered a URL rewrite, making the process of going back in history in the browser tedious.

Could we set a reasonable timeout so that the URL rewriting only happens if a customer hasn't entered any characters in the last X ms? If so, what would be a good value for this?

#### Screen capture


No file or test is named in the issue. Start at the Instant Search URL-rewriting entry point and reproduce the behavior by entering “posts” one character at a time. Done means URL rewriting waits until typing has paused for an agreed interval, so browser Back does not require stepping through every character.

Written by the indexing model from the issue text.