Support kXR_query kXR_Qprep

[alrossi]

As in EOS to allow clients to query the status of prepare requests for tape-backed files. I have documented it in the CTA documentation (see section 3.2, QUERY_PREPARE):

( https://gitlab.cern.ch/cta/CTA/blob/master/doc/EOSCTA.pdf)

The response is a JSON object which is described in the doc.