Bugfix/unique node ownership

[jstuczyn]

@futurechimp Before merging it, let's have a discussion about what to do with the migration. I see two options here:

1. Don't do anything, the problem should solve itself as people unbond and rebond to update.
2. During migration iterate through all bonded mixnodes and (assuming they are in order which I'm not sure of), use the first entry as ownership and insert that data.

[futurechimp]

Let's talk about it tomorrow.