Iterate multiple times over the same set of bonds when unbonding

[angbrav]

When a user unbonds a number of tokens = amount, the system iterates over a set a bonds and creates an unbond for each of these bonds until we reach amount. After this process, we create a new bond with -amount. This is a problem, as if there is a second unbond, we will iterate over the same set of bonds, which are supposed to have been unbonded already. We need a solution that avoids iterating over already unbonded bonds.

We have discussed two solutions:

• Simplest solution: update the amount of each bond. This means to reduce to 0 for all but the last iterated.
  We are destroying info, but seems not to be a problem because we use unbonds when slashing. Only concern: discrepancy between total_deltas and bonds.

• An alternative solution could be one similar to total_unbonds.

We have agreed to try the simplest solution.