[operator contract] Allow the staker to unset the operator.

[ethanfrey]

Right now, a staker must set an operator before any commitments can flow through them on this contract.
They can later update the operator that their commitment flows through, triggering a "restaking".
However, they currently cannot signal that they are stopping operations.

A staker should be able to unset the operator (deleting that key), while keeping the reverse mapping of operator -> staker, so they can still be slashed during the unbonding period.

The existing stake to that operator should enter the unbonding period locally, and once that period has passed, they can make a call that will trigger ReleaseStake on the provider (meaning it is fully unbonded, and can release all the way up the chain).

This will require two new ExecuteMsg variants:

• unset operator
• release unbonded (or such)

You may want to add new queries as well to expose this information.

This is also needed to test #61 as it will be the first real way we will trigger release stake in a normal contract flow.

[ueco-jb]

while keeping the reverse mapping of operator -> staker, so they can still be slashed during the unbonding period

Ah, that makes sense. I remember you mentioning that the reverse mapping is important but I finally understood why.