Shutdown socket gracefully. Simplify server cleanup.

[eryx67]

This patch add two things:

• shutdown socket before closing
• change custom forkFinally with the same function from SlaveThread to
  simplify server cleanup on shutdown. Without it user need to develop a custom solution
  for killing incoming connections or they will wait for client disconnections.

[k0001]

Thank you @eryx67. The shutdown change is good, we should add it. Let me think about slave-thread a bit more though: I think it is a good idea in principle, but I am a bit hesitant about adding dependencies to this library.