This reverts commit 5cda9ec6.

Integrates XRootD logging

Not required by new SSI interface as endpoint is specified in the
workflow not in a configuration file. But it is still needed by
cta-admin.

Use in the EOS workflow/bash scripts is deprecated. However it's useful
to keep the script around for now to test the SSI workflows without needing
to invoke EOS.

Now the submodule does not provide a CMakeLists.txt file, this is now
provided by the main project (eos_cta/CMakeLists.txt)

As EOS is parsing stderr to get the archive ID

Replaced set_target_properties( .. .. INSTALL_RPATH ..) with set_property (TARGET .. APPEND PROPERTY INSTALL_RPATH ..)

Compile switches now applied to each target separately, cta_admin has
instrumentation switched off.

Zero-length metadata now resolves to a valid metadata messages, with
RSP_SUCCESS and no text message.

This allows specifying different protocol buffers for EOS and CTA Admin
messages, both of which must be processed by the same XRootD plugin in
the CTA Frontend.

This library contains the protobuf which defines the API for the CTA Frontend.
This will define messages for the CTA admin tool as well as EOS so EOS should
not be part of the library name.

XrdSsi headers will be merged into master branch, but move into private/

This is deprecated but is useful for testing: it allows eoscta_stub to
be dropped in in place of the "cta archive" command in the WFE in the
current test setup.