Fix to ODIDDataSource for calculating datarate for negative streams (cvp2ri-433)

When doing the byterate calculation (for adjusting chunk size in live
 live streaming), the total duration was taken from the last entry of the
 content file regardless of direction. For negative-rate content this
 duration needs to be taken from the first entry.

Fixed the ODIDDataSource to pass the direction into the duration extraction
 functions and updated the functions to factor in duration.

Haven't performed any tests as-of-yet.
Merge pull request #30 from cablelabs/cbl_cpratt/cvp2ri-433

Fix to ODIDDataSource for calculating datarate for negative streams (cvp...