Delay the release of collateral by validators in leave

[adlrocha]

A call to leave from a validator releases the validator's stake immediately in the current implementation. This is possible because there's no slashing. Once slashing is implemented in the system, we can't release validator's stake immediately, and we should include a delay for the validator to be able to recover their stake to prevent a validator from being able to run with their funds before any active complaint/dispute for slashing it may be involved is settled.