Document the bonds/bounties design and mechanics

[thec00n]

Issue Type

[ X ] bug report
[ ] feature request

Current Behavior

With #698 exit bounties are introduced to provide incentives to process the exit queue. As commented here #698 (comment) currently there is no specification available that provides a rationale the different flows where bounties and bonds are transferred between parties. It is therefore difficult to reason about the correctness and to evaluate the security properties of the system. Further it be useful to explain to users why they have to pay or how they might claim bonds or bounties.

Expected Behavior

Create a comprehensive spec on each of the flows where bonds and bounties are transferred between parties during a SE/IFE and provide a rationale for the incentives that are provided. Ensure that tests are comprehensive and that they cover all scenarios and potential attack vectors.