Fix multiperiod switching

[bbert]

This PR is fixing some issues when switching periods, due to controllers lifecycle and seeking event handler.
For example it fixes the following javascript error when switching period:

BufferController.js:398 Uncaught TypeError: Cannot read property 'waitForUpdateEnd' of null
    at pruneAllSafely (BufferController.js:398)
    at Object.onPlaybackSeeking (BufferController.js:386)
    at EventBus.js:124
    at Array.forEach (<anonymous>)
    at Object.trigger (EventBus.js:124)
    at HTMLVideoElement.onPlaybackSeeking (PlaybackController.js:499)