Fix #1759 (regression) resume correctly after seek

[JontyUsborne]

Fixes #1759 caused by 930717e.

Original reason for 930717e was a problem were segment lists were updated out of order such that the previously fetched index was sometimes no longer valid. For instance, if a paused player drifted outside of the rewind window, when seeking back, the getSegmentByIndex and getIndexForSegments functions would be using different segment lists.

Moving updateSegments fixed the issue, but caused problems for other streams where the segment list needs to be updated on every getSegmentRequestForTime call.

6650178 now works, but the whole thing could probably do with refactoring and the first updateSegments should probably go somewhere else.