rebase onto performance-timeline-2, etcetera

[cvazac]

included changes:

• OWS in the ABNF
• explanation for omission of meaningful startTime
• duration as a number, not DOMHighResTimeStamp
• renamed interface from ServerEntry to PerformanceServerTiming (https://github.com/w3c/server-timing/issues/3)
• removed bits about controlling the buffer from javascript
• add sequence of PerformanceServerTiming objects to the PerformanceResourceTiming object as serverTiming
• fixed example formatting
• fixed interface sample code
• PerformanceServerTiming no longer extends PeformanceEntry
• rename duration to value

cc @igrigorik @yoavweiss

[yoavweiss]

NavTiming addition also LGTM