refactor PSGI logging

This MR follows !336 (merged) and refactors it to avoid some code duplication and simplify code paths

All PSGI loops are now supposed to be created by the psgiAdapter method. This method takes a sub like

sub {
  my $req = shift; #PSGI::Request object
  return $self->handler($req);
}

and turns it into a "real" PSGI app (that takes an env hashref). It takes care of instanciating the Request object and setting up the logger with request context, and logs the new request at info level