ttools: add infrastructure for upload matches to CDS Xmatch service

Library support is added to allow tables to be uploaded to the
CDS Xmatch service in chunks and the result assembled for presentation
to the user.
ttools: add cdsskymatch task

This uses the CDS X-Match service for crossmatching local catalogues
against CDS-hosted tables, specifically VizieR and SIMBAD.
ttools: note bug in cdsskymatch find=best-remote mode

Ensure the bug is obvious to users, and add hooks to do something about
it when I have the relevant information.