in_xbee: Improvements in payload queuing
in_xbee: Improvements in payload queuing

- Omit extra bytes when queuing a MessagePack payload
- Accepts and queues multiple MessagePack payloads in a XBee packet

Signed-off-by: Takeshi HASEGAWA <80595b5c49522665976d35e515d02ac963124d00@gmail.com>

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