For SegmentTemplate, only include available segments in DVRInfo
Created by: LloydW93
This PR resolves #1362 (closed). Currently, the DVRInfo event only uses the current available time and the dvrwindowlength to derive the available start/end times.
By checking the AST of the first segment currently available and the AET of the last segment currently available, we ensure we only report times that are actually currently available.