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

Updates to XRootD SSI to fix bug with using SSI + OFS similtaneously

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.

Required to read user name from XRootD key

Merged into XrdSsiCtaServiceProvider.cpp as it does not need to be
included anywhere else.

Necessary in order to make ServiceClientSide reentrant

The implementation of Stop() is simply { return false; } so this
function is superfluous. It's not possible to shut down the service
on the client side except when completely closing down the client.

Using Promises and Futures

The framework does not throw exceptions to the client, instead it puts
the exception in an Alert and calls the AlertCallback to log it.

Alert objects are self-destructing after they are sent (or cancelled)

* EOS->CTA is a Notification message
* CTA->EOS is a Response message (sent as Metadata)
* Optional Alert for extra log messages (sent as Alert)

To better understand the order in which objects are created and
destroyed, and what their lifetimes are.

There needs to be some proper synchronisation here, as Finished() must
be called before UnBind(). Sleep is a stopgap, I have raised an issue
with Andy.