livehttp: follow draf on segment duration

livehttp: follow draf on segment duration

Currently Draft on HTTP live streaming says that segment should be upto
TAGETDURATION length. This change makes sure segments are upto
seglen+10% length, it doesn't mind if they are left quite a lot shorter
(like 10s keyint and 10s seglen can create segment with 2 sec long if
next segment is 10s).

Livehttp-module buffers blocks until header-block turns up. Then it
will check if current buffer would fit on current segment without making
it overflow, or starts new segment if it goes over.