Prevent exception caused by $service.stop

Since `poll_one` is a blocking call, when service is stopped it throws
with `X::AdHoc` due to -1 returned by `zmq_poll`. Unfortunately,
`poll_one` doesn't use `zmq_die` and therefore there is no way to
determine if the error is caused by socket closing or not. For this
reason this PR relies on `$closer` value to find out if the exception
must be rethrows or dropped.

PR arnsholt/Net-ZMQ#22 is submitted to replace plain `die` with
`zmq_die`. But until then this PR should be ok get things straighten
out.

lib/Cro/ZeroMQ/Internal.pm6

@@ -105,6 +105,7 @@ role Cro::ZeroMQ::Source::Impure does Cro::Source does Cro::ZeroMQ::Component::I
  start {
  loop {
  last if $closer;
+ CATCH { default { .rethrow unless $closer } }
  my $event = poll_one(self!socket, 100, :in);
  if $event > 0 {
  $messages.emit: Cro::ZeroMQ::Message.new(parts => self!socket.receivemore);
@@ -133,6 +134,7 @@ role Cro::ZeroMQ::Source::Pure does Cro::Source does Cro::ZeroMQ::Component::Pur
  start {
  loop {
  last if $closer;
+ CATCH { default { .rethrow unless $closer } }
  my $event = poll_one($isocket, 100, :in);
  if $event > 0 {
  $messages.emit: Cro::ZeroMQ::Message.new(parts => $isocket.receivemore);