signalc: handle incoming signal messages

GIVEN a number that has been registered and a running toy demo app

• WHEN a message is sent to the number from another signal user
• THEN the message is printed to STDOUT by the toy app

GIVEN the above

• WHEN 2 messages are sent to the number from 2 different people at roughly the same time
• THEN the messages are printed to the screen
• AND signalc does not crash or bork the protocol store (due to concurrency errors)