Modifed WorkerIdleTTL logic

Before, WorkerIdleTTL would cause a host goroutine to be killed
when no new links were inserted into the goroutine in WorkerIdleTTL time.
The new behavior is, WorkerIdleTTL will cause a host goroutine to
be killed when the host goroutine has not crawled a page in
WorkerIdleTTL. This allows a host goroutine to build a large
queue of links and have time to crawl them all, rather than being cut
short by the old behavior.