groot+xrootd: investigate+fix abysmal performances with rtree.Reader

[sbinet]

the new rtree.Reader implementation blows ROOT out of the water.
but that's for local files.

when reading a remote file over xrootd (but, probably, also over http[s]), performances aren't that good.

there are 2 parts:

• performances of xrootd can be improved
• I/O scheduling (rbasket and bkreader) for remote files should be customized (e.g.: to not "fetch-storm" baskets)