Ensure a legacy validator can't accept new delegations

[rigelrozanski]

When a validator unbonds it's last self delegation token it should no longer be able to accept new delegators - this said, the validator object still needs to persist as the existing delegators need to be able to withdraw any tokens they still have with this (now-unbonded/unbonding) validator.

[cwgoes]

As in, we always want to enforce a self bond greater than zero?

[rigelrozanski]

well, yes and no - this is going to be a funny one - because we need to have validators with zero voting power for launch specifically - BUT I think how we can accomplish this is be only checking when the self-delegation is unbonding. so at launch we can have validators with no self-delegation who still accept new delegations, which is fine, but as soon as sending is enabled, and the presumably self-delegate to themselves, at that point, if they unbond their last self delegated atom, they would be barred from accepting new delegations.

It's effectively a way of signalling that the validator is shutting down

[cwgoes]

I guess a zero self-bond validator would have no reason to un-self-bond, so we can safely use it as a signaling mechanism - cool - implementation proposal seems reasonable.

[alexanderbez]

Blocked on #1676

[rigelrozanski]

is this actually blocked on 1676?

[alexanderbez]

@rigelrozanski I tagged the wrong issue -- my bad.