Renamed CTA/tapeserver/castor to CTA/tapeserver/castor_checkout in preparation for the moving the sub-directory CTA/tapeserver/castor/castor up the directory tree

The ProcessForker can impose "entire lifetime" or "per mount"
refreshes of CASTOR configuration-parameters.  There is no
need for the tapeserverd parent process to do this and send
the results over the wire.

The C++ namespace castor::tape::tapeserver::drive now matches its
directory structure.

Tapeserverd now support file://, root:// rfio:// and radosStriper:// URLs, on top of heuristic protocol guessing from the hostname:[/]path synthax.
The rados striper version is not yet fleshed out yet.
The xroot client used for this is the latest object oriented client (XrdCl::File).

Please note that the ProcessForker class does not as yet do anything.
I am making this commit due to all of the reshulling work I have had
to do to get this far.

MountSession now provides the zmq context for everyone in the forked-session and closes it in its destructor

Added a mechanism to let know if the MountSession managed to remove the tape from the drive or if it needs to be put down

The MountSession is now used in both its unit tests (in its new form) and in the tapeserver's
binaries.

when failing to signal the end of an empty session.

As a step closer to removing the need for a separate castor/tape
directory, I have moved castor::tape::legacymsg to
castor::legacymsg.