Limit req: fixed delaying subrequests.

Limit req: fixed delaying subrequests.

Since limit_req uses connection's write event to delay request processing,
it can conflict with timers in other subrequests.  In particular, even
if applied to an active subrequest, it can break things if wev->delayed
is already set (due to limit_rate or sendfile_max_chunk), since after
limit_req finishes the wev->delayed flag will be set and no timer will be
active.

Fix is to use the wev->delayed flag in limit_req as well.  This ensures that
wev->delayed won't be set after limit_req finishes, and also ensures that
limit_req's timers will be properly handled by other subrequests if the one
delayed by limit_req is not active.