* _check_resolved_address() is implemented with getaddrinfo() which is slow
* If available, use socket.inet_pton() instead of socket.getaddrinfo(), because
  it is much faster

Microbenchmark (timeit) on Fedora 21 (Python 3.4, Linux 3.17, glibc 2.20) to
validate the IPV4 address "127.0.0.1" or the IPv6 address "::1":

* getaddrinfo() 10.4 usec per loop
* inet_pton(): 0.285 usec per loop

On glibc older than 2.14, getaddrinfo() always requests the list of all local
IP addresses to the kernel (using a NETLINK socket). getaddrinfo() has other
known issues, it's better to avoid it when it is possible.

event loop or a transport is not explicitly closed

already know the traceback of the source.

The handle may be more revelant, but having 3 tracebacks (handle, source,
exception) becomes more difficult to read. The handle may be preferred later
but it requires more work to make this choice.

In debug mode, BaseEventLoop._run_once() now sets the
BaseEventLoop._current_handle attribute to the handle currently executed.
In release mode or when no handle is executed, the attribute is None.

BaseEventLoop.default_exception_handler() displays the traceback of the current
handle if available.