Make the network resource queuing delay whiskers more subtle

Follow up from https://github.com/firefox-devtools/profiler/pull/1929#issuecomment-485044719, cc julienw

Rational:

* The whiskers are helpful indicators but should have less visual weight than the main request/response bar
* The design should be closer to waterfall charts provided by other tools

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-148)