Support for live-mode chunk calculation

Live streaming of B-MP2PS_N-1 revealed some interoperability issues with
 low-bitrate content. e.g. the default chunk size of 1572864 (1536K) contains
 more than 1 second's worth of data on multiple occasions when streaming
 this content.

Remedy:

 The MediaEngine no longer maintains the policy for the default chunk size.
 If it's not set in rygel.conf, "0" is passed to the DataSource which
 establishes the default value.

 For static data sources, the MediaEngine will use the default chunk size
 when "0" is passed to the constructor. For live sources, the chunk size
 is now set to 1/2 the average bytes-per-second for the source content.

Since there was yet another place where content size was being utilized,
 added ODIDUtil.file_size().

Tested live streaming with B-MP2PS_N-1. Playback was smooth with the reduced
 chunk size.