sets liveStartTime only if it has not been set yet or new value is bigger than previous one
fixes jasmine tests
fixes possible InvalidStateError when MediaSource.endOfStream is called
allows executiting pending requests even if nextFragmentRules return null
adds an explicit test for undefined value when returning result of rule execution
remove rounding when retrieving currentTime in SameTime RequestRule
adds a threshold when searching for a segment for a time in FragmentModel.getRequestForTime
allows calling endOfStream only for the last stream in multi period mpds