fix (most) random crashes when receiving UDP traffic

When receiving many (UDP) packets RIOT crashes both on native and on the msba2.
It turns out that lost messages in the network stack are not well handled well: If a queued message gets replaced by a new one, [sender](https://github.com/benpicco/RIOT/blob/master/core/msg.c#L254) in `_msg_receive` points to invalid data, RIOT crashes.

This pull request works around this issue by hooking up the message queue in ip.c that is defined, but never used. It also introduces a similar queue for the udp thread, so now up to 64 packets can be received   at roughly the same time before RIOT crashes.