Implement HTTP session support in REST server and client for default auth

The REST server (and java client) currently does not implement sessions.

While is not necessary for the REST API to work, implementing sessions would be a big improvement in throughput and resource usage.

• It would make load balancing with sticky sessions possible
• It would save the overhead of performing authentication for each request

The gains are particularly big when using SPENGO:

• The full SPENGO handshake can be skipped for subsequent requests
• When Knox performs SPENGO authentication for the proxied client, it access the identity store each time. When the session is set, this step is only perfomed on the initial request.

The same change has resulted in spectacular performance improvements for Phoenix Query Server when implemented in Avatica.