Add option to force live latency to a given number of seconds

Created by: AxelDelmas

There's currently no easy way to specify how many seconds of latency you want for live streams:

• setLiveDelayFragmentCount will result in random latency if segments have variable durations
• if I understand it right, the maximum latency is limited to a half manifest duration

I introduced an additional setter which specifies the target latency in seconds.

• If set, this target latency will take precedence over setLiveDelayFragmentCount, DVRWindowSize and so on
• if not set, the current behavior is used