v0.4.0: Major overhaul of concurrency system

This release comes with a brand new concurrency system,
originally a fork of std.concurrency which was modified
to support the notion of channels (inspired from Golang's API).
It should not interact (or conflict) with std.concurrency anymore.

Other major changes:

• Threads can now be shut down explicitly;
• Request mismatch are now handled gracefully (the node won't crash);
• There is a new module, geod24.Registry, to allow easily mocking a router / DNS;
• The std.concurrency priority messages have been removed;
• Localrest can now use a custom serialization policy, with vibe.data.json being the default;
• An comprehensive error message will now be printed out when a node dies (including instantiation location);
• Nodes responding to dead peers will no longer crash;
• Control interface: Shutdown can now accept a custom callback for user-defined cleanup;
• setTimer (similar to Vibe.d) and schedule have been implemented for task management;
• Control interface: Nodes can now be restarted;
• Client error (e.g. peer dead, unimplemented methods) are now separate from error response (e.g. wrong parameter);
• Various improments around Tid to make type mismatch harder;
• The scheduler can now be used from the node constructor;
• All operations now default to a timeout of 5 seconds (previously: unlimited);
• Self connecting nodes (nodes that connect to themselves) should now work properly;