bitbake: xmlrpc transport has identification token

In order to be able to identify different clients over a
stateless XMLRPC connection, we add a custom header named
Bitbake-token, which identifies each client.

We refactor the rest of the code to use the new transport.

Based on a patch by Bogdan Marinescu <bogdan.a.marinescu@intel.com>

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
bitbake: xmlrpc remote server

Added code in XMLRPC server that creates a stub local server
for a client-only connection and is able to connect to
a remote server, and receive events from the remote server.

Added the option to start a client with a remote server in
bitbake.

Original code by Bogdan Marinescu <bogdan.a.marinescu@intel.com>

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
bitbake server: create common server infrastructure

In an attempt to minimize code duplication, create
clear interfaces, and maximize code reuse through OOP,
bb.server adds base classes for the BitBakeServer,
BitBakeServerConnection and actual server implementations
instructed in particular server types.

These classes document the minimum interfaces that the
derived classes must implement, and provide boilerplate code.

Changes to None, Process and XMLRPC servers as to use
the common server infrastructure.

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>