Consequent publish data point operation return "double spending" node error

[greenhat]

Discovered in #61

After the first successful publish datapoint node request, the second publish request (in 30 sec) end up with "double spending" node error.
The reason for the second request is that in 30 secs there is no published box in wallet scans, thus publishing again.

There are the following options on how to fix this behavior:

• ignore this particular node error, eventually, (a few minutes) wallet scan will return the published box;
• save in-memory status that datapoint was sent to the node in this epoch;