Thread safe immutable data exchange

[arnobaer]

Introduce thread save immutable data exchange mechanism (exchanging only deep copies) using methods get(key, default=None) and set(key, value)

>>> def run(p):
...     for i in range(42):
...         value = p.get('count', default=0)
...         value += 1
...         p.set('count', value)
>>> p = comet.Process(target=run)
>>> p.set('count', 0)
>>> p.start()
>>> p.join()
>>> p.get('count')
42