datatransfer: use non-blocking read on rx side

PeerConnection eventloop was blocked on receiver side
on a read() call.
Use a polling method to let possiblity to event msg to
be handled.
This fix non-working data transfer cancelation.

Change-Id: I602b6ba27091e59ad0978f4e4023204d39ee8223
Reviewed-by: Olivier Soldano <olivier.soldano@savoirfairelinux.com>

src/peer_connection.cpp

@@ -578,9 +578,12 @@ PeerConnection::PeerConnectionImpl::eventLoop()
                 throw std::system_error(ec);
             });
         handle_stream_list(outputs_, [&](auto& stream) {
+                if (endpoint_->waitForData(10, ec) > 0) {
                     auto size = endpoint_->read(buf, ec);
                     if (!ec)
                         return size > 0 and stream->write(buf);
+                } else if (!ec)
+                    return true; // continue on msg handling
                 throw std::system_error(ec);
             });
     }