New profiles trigger a bunch of places queries for highlights

[Mardak]

Noticed when testing #3695. Regressed by #3675:

activity-stream/system-addon/lib/HighlightsFeed.jsm

Lines 150 to 154 in d8f0b4a

	case at.PLACES_BOOKMARK_ADDED:
	case at.PLACES_BOOKMARK_REMOVED:
	this.linksCache.expire();
	this.fetchHighlights({broadcast: false});
	break;

New profiles create some bookmarks causing highlights to expire the cache. We end up requesting new highlights but also get none because they're too recently bookmarked (probably default bookmarks).

@piatra

[piatra]

I already created #3724 to handle this.

[Mardak]

Ah doh. too many issues ;)