XrdCl: Optimize performance of xrdcp uploads

CERN->Wigner before:
]==> xrdcp ./a048e67f-4397-4bb8-85eb-8d7e40d90763.dat root://p05614923h06774.cern.ch//tmp/`uuidgen`
[1000MB/1000MB][100%][==================================================][40MB/s]

CERN->Wigner after:
]==> xrdcp ./a048e67f-4397-4bb8-85eb-8d7e40d90763.dat root://p05614923h06774.cern.ch//tmp/`uuidgen`
[1000MB/1000MB][100%][==================================================][111.1MB/s]

CERN->CERN before:
]==> xrdcp ./a048e67f-4397-4bb8-85eb-8d7e40d90763.dat root://eospps.cern.ch//eos/ljanyst/cptest02/`uuidgen`
[1000MB/1000MB][100%][==================================================][76.92MB/s]

CERN->CERN after:
]==> xrdcp  ./a048e67f-4397-4bb8-85eb-8d7e40d90763.dat root://eospps.cern.ch//eos/ljanyst/cptest02/`uuidgen`
[1000MB/1000MB][100%][==================================================][104MB/s]