DNS Instrument only records the final request in a redirect chain

[englehardt]

Originally reported in #989, but I noticed it myself while playing around with some crawl data recently. The problem stems from the listener we're using to record DNS entries.

OpenWPM/Extension/src/background/dns-instrument.ts

Lines 29 to 44 in ab5fac2

	/*
	* Attach handlers to event listeners
	*/
	this.onCompleteListener = (details: WebRequestOnCompletedEventDetails) => {
	// Ignore requests made by extensions
	if (requestStemsFromExtension(details)) {
	return;
	}
	const pendingResponse = this.getPendingResponse(details.requestId);
	pendingResponse.resolveOnCompletedEventDetails(details);
	this.onCompleteDnsHandler(details, crawlID);
	};
	browser.webRequest.onCompleted.addListener(this.onCompleteListener, filter);
	}

The onCompleted event is only fired after redirects are resolved. Instead, we want to use an event that is part of the redirect loop. e.g., onHeadersReceived. I tested this and it works well, but I still need to add some tests.