xrootd transfer re-implementation
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This merge request is automatically copied from !653 (merged) orginally made by @dcameron
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Re-implementation of xrootd DMC to do file transfer based on xrootd copy library instead of posix library. This improves transfer performance (BUGZ-3695) and adds support for zip extraction (BUGZ-3784). A new DataPoint method Transfer() was added to support transfer done internally by a DMC.