Avoid unnecessary requests to the time sync server

Created by: dsilhavy

Environment

• Link to playable MPD file: https://livesim.dashif.org/livesim/mup_30/testpic_2s/Manifest.mpd
• Dash.js version: 3.2.1
• Browser name/version: all
• OS name/version: all

Observed behavior

Play the stream and wait for MPD udaptes. After each MPD update a request to the UTC sync server is done: https://time.akamai.com/?iso&ms . Expect for the first request, the response is ignored.

Expected behavior

We should avoid unnecessary requests. DASH-IF 4.3 states: 4.7.3: "Clients should not access the UTCTiming server more frequently than necessary".

Suggestion: Only do a single sync at startup. Behavior would be similar to now since we ignore the response anyways. Clarify with @haudiobe what the best behavior is.