Make sure pageview hit sent before event hit in Google analytics tracking

[newgene]

See this warning from Google Analytics:

Relevant code here:

biothings.api/biothings/web/analytics/channels.py

Lines 51 to 55 in bba25ac

	for i in range(0, len(events), 20):
	yield HTTPRequest(
	'http://www.google-analytics.com/batch', method='POST',
	body='\n'.join(events[i: i+20])
	)

Since we now send hits in batch with async, we should make sure event and pageview hits send together if they belong to the same pageview.