Option Name:url_rewrite_concurrency
Default Value:url_rewrite_concurrency 0
Suggested Config:

	The number of requests each redirector helper can handle in
	parallel. Defaults to 0 which indicates the redirector
	is a old-style single threaded redirector.

	When this directive is set to a value >= 1 then the protocol
	used to communicate with the helper is modified to include
	a request ID in front of the request/response. The request
	ID from the request must be echoed back with the response
	to that request.