Implement shared memory

[pietroalbini]

Because the botogram runner works in a multiprocessing environment, bot developers haven't any native way (excluding external services) to share data between the different workers.

Shared memory provides a simple and reliable way to do that. The API is simple, and allows synchronization of any picklable object. It will also implement a way to initialize shared memories.

@bot.init_shared_memory
def init_shared(shared):
    shared["messages"] = 0

@botogram.pass_shared
@bot.process_message
def process_message(shared, chat, message):
    shared["messages"] += 1

• Implement the basic structure of the shared memory (not in the runner)
• Implement shared memory in the runner
• Fix shared memory not working on Windows (issue Cannot start shared-memory branch on Windows #3)
• Implement init_shared_memory
• Document the whole thing

This issue tracks the progress for this feature.

[pietroalbini]

Shared memory landed in master right now.