staking: change compute_unbonding_height api

[erwanor]

This PR modifies the staking module logic responsible for determining the unbonding schedule, in particular it:

• returns the current height if a pool is unbonded
• change the compute_unbonding_height API to return an Option<u64> to allow callers to detect an unbonded pool

One tricky aspect of our unbonding mechanism is that we want to:

1. Bind unbonding notes to the economic effect of validator misbehavior for some window of blocks
2. Leave it to the application to decide when a note is allowed to be claimed

One thing to note is that this mechanism works relative to the current bonding state of the pool. This is generally fine for the protocol security because this leads the application to "overshoot" the penalty window range since it cannot know the exact moment a note was allowed to unbond (since bonding the state could have changed). This means that when processing a claim, we detect if the pool is unbonded, and if so, we set the "allowed unbonding height" to the current height.