Update multi period management

Created by: nicosang

Hi,

the goal of this PR is to update management of multi period streams.

First, when a switch stream occurs, streamProcessors array is destroyed. It's the reason why abrController don't have to keep references on those streamProcessors.

Then, after a stream switch, executedRequests array (in FragmentModel) was not cleaned. That's why if we seek back to the initial stream at an already played time, the getNextRequestTime returns a request at the time of the end of the last executed Request...not the wanted time. A test can be done with the stream https://dash.akamaized.net/dash264/TestCases/5a/1/manifest.mpd : play the initial stream (about 10 seconds), seek in the second stream (for instance at 4mn30s), and then after 5 seconds seek to the initial stream at 10 seconds. The stream plays from about 40 seconds not 10 seconds (reproduced on nightly build page). In brief, when a switch stream occurs, the executed requests array need to be cleaned : the source of the video is changed, buffered data are reseted.

Nico