Using Promises and Futures

Simplifies code and avoids unnecessary string copy construction

Handling of multi-block responses and streaming is not yet implemented.

Renames private member variables. Changes to some constructor arguments.

First character of XrdSsi metadata is overwritten with zero, see
https://github.com/xrootd/xrootd/issues/537
This workaround adds a dummy character to the front of the metadata on
the server side and removes it on the client side.

Now waits n=30 secs for any outstanding requests to finish before
giving up and executing a forced shutdown.

Metadata can be used for short replies (less overhead than a full reply)
or to provide information about the reply which will follow (e.g. number
of records, response size, etc.)

Alerts can be used to signal the client about the server state.

Protobufs for different message types will now be handled by template
specialization