anymore.

The event-loop has now been extracted inside its own Io::Service
component which allows us to reuse it.
A new Io::ServiceGroup class has been introduced for multi-threaded
I/O. a ServiceGroup will spawn multiple Io::Service inside their own
thread. Every instance of Io::Service also has its own Io::Handler, the
handler is not shared between io threads.

A route can now be bound to a non member function. Also changed the API a litle bit to avoid big code duplication

Http::Endpoint: added a threaded serving mode that will spawn an acceptor thread rather than running in the same thread

Introduced a lock-free MPSC queue between the user and I/O thread to make sure that all I/O operations happen in the dedicated I/O thread

Listener: requestLoad does not return a resolved promise anymore but instead directly returns the result

Async: implemented a whenAny primitive that returns a Promise that will be resolved when one of the Promise is resolved

Async: added a way to rethrow an exception and then propagate the exception along to an other promise

Do not call Request::associatePeer anymore until the mysterious lock contention has been found and resolved

Basically, this commit brings a brand new Response API to the party:
   - Response::send() can be used to send a fixed-size content and
     will use the classic Content-Length header
   - When content size is not known in advance, a Stream interface can
     be used to stream the response. A ResponseStream is obtaining by a
     call to the Response::stream member function. After that, headers
     will be immutable and data can be streamed a-la-ostream with
     operator<<

Also fixed a bunch of issues:
    - Flags<T> now works correctly and correctly zero-initializes the
      value in the default constructor
    - Fixed a nasty bug in Poller that went unnoticed because the Tag
      constructor was not marked explicit