xroot: improve performances #920

this is possibly related to #399.

consider:

$> time root-dump root://ccxrootdgotest.in2p3.fr:9001/tmp/rootio/testdata/SMHiggsToZZTo4L.root > /dev/null

real	1m8.557s
user	0m14.863s
sys	0m3.562s

running over the same file but with http[s]:// (which, currently, downloads the whole file and then serves it locally):

$> time root-dump https://cern.ch/binet/big-file.root > /dev/null

real	0m5.454s
user	0m6.156s
sys	0m0.228s

a factor 10x is left on the floor.
(ok, not the same machine, on different networks, etc... but still)