mpdparser: Parse MPD Anchor "t" and "period" parameters
Submitted by Seungha Yang
Link to original bug (#775757)
Description
ISO/IEC 23009-1:2014 Annex C.4 defines MPD Anchor which is to indicate
start time offset, period, and etc.
Basically, the syntax of MPD Anchor follows Media Fragments URI 1.0
from key-value pair parsing point of view.