It appears that not all Apple implementations follow the HLS guidelines.
While the DEFAULT=NO for an audio track should be optional and default
to NO, in practice native HLS players Safari and iOS devices treat the
missing DEFAULT as a MAYBE.
Fixes#1169
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