The number of preserved segments outside live window can be
configured using flag --preserved_segments_outside_live_window,
which is default to 50, i.e. 5 minutes for 6s segment.
Note that the segment removal will be disabled if it is set to 0.
Only HLS live playlist and DASH dynamic MPD are affected by this flag.
- Also add end to end tests.
Fixes#223.
Change-Id: I8a566efebe2f1552c7d9509ab017bade5a4a1c98