Refactor SegmentBaseLoader to use shared segment loading code

Created by: davemevans

Recently, all XHR requests for manifests, xlink and 'normal' fragment loading were refactored into a base loader (XHRLoader), with a view to removing duplication and, in future, facilitating alternative transports.

SegmentBaseLoader has always done its own thing for some reason, and it should probably be transitioned to the standard shared code if at all possible. At first glance, it looks like it may be.

This isn't an area I'm familiar with since our streams, and most others I am familiar with use the FragmentLoader. Does anyone have in-depth understanding of SegmentBaseLoader?