bitbake: knotty, xmlrpc: add observer-only mode

bitbake: knotty, xmlrpc: add observer-only mode

I add an observer only mode for the knotty UI and
the XMLRPC server that will allow the UI to register
a callback with a server in order to receive events.

The observer-UI is able to send read-only commands to the
server, and also is able to register as an event handler.

Read-only commands are the commands that do not change
the state of the server and have been marked as such in
the command module.

The observer can switch to a full client if it calls addClient
at any time, and the server has no other client running.

(Bitbake rev: 4de9ee21f1fa4d04937cc7430fb1fc8b7a8f61e2)

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