Thread and connection limit

[cheme]

Some mechanism are needed to limit thread :

• by custom accept rules, accept should limit number of peer, that is nice first step to determinist number of threads, yet currently accept return bool an refused node are considered 'blocked' which is not the correct behaviour, so accept need to evolve to have more suitable return type (or another function to accept_new_thread).
• by lib management (need more work), use rules to define this limit, then plug some management in :
  • server : need a way to remove some connect (currently not easy)
  • peermanager : should manage some count

[cheme]

New thread model, option to run on thread pools for server and client.
Currently only implemented for client, server could run on single thread as current alternative.