What is the lightest solution to creating a multiprocess architecture with state shared across all processes
Yes. The database most likely involves the harddrive, and the harddrive is the slowest part of any computer so switching away from using the harddrive will probably have performance benefits.
I would go with zeromq / zmq. Its a message oriented framework that supports several communication patterns. For instance PUB/SUB or REQ/REP etc. More examples here
zmq is cross platform and its amazingly fast.