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>