Allow multiple vesting locks per account

[aaronc]

No description provided.

[JimLarson]

FYI, in the Agoric fork we had the following issue for allowing multiple locked-up grants to the same PeriodicVestingAccount: Agoric/agoric-sdk#3956

It was implemented in the following PR: agoric-labs#136

[tac0turtle]

@JimLarson it seems agaric has put a lot of work into vesting, Id love to come up with a plan of upstreaming this work since most the vesting issues in the sdk seem to be solved with your work.

[alexanderbez]

Yes, please! Getting a clawback mechanism would be super beneficial to have in the SDK's vesting account types.

[JimLarson]

Great! I'd love to chat about upstreaming plans.

BTW, I found an issue with agoric-labs#136 (that also affects Clawback vesting accounts). The current vesting account bookkeeping for delegated tokens assumes that the unvested amount is monotonically decreasing. If we allow new vesting grants into existing accounts, then slashing can lead to future grants being unlocked prematurely. One solution is to add a new feature to x/staking to get a balance of the current bonded and unbonding amounts, where the result is cached for efficiency but invalidated after a new slashing event.

See Agoric/agoric-sdk#4300

[tac0turtle]

closing this in favour of #14900.