Add a function to pbo_delegation_pool.move that allows the vesting schedule to be changed if there are no principle stakeholders

[isaacdoidge]

pbo_delegation_pool.move does not currently support modifying the vesting/lockup schedule of a DelegationPool once it has been created. At genesis, we created many pools with the PBO Plan 1 vesting schedule, but only ended up using 10 of them for this purpose. We now need to allocate another 10 (or so) pools for PBO Plan 2, and also need to be able to onboard the nodes of operators who participated in the final investment round before the exchange listing. Both of these requirements involve custom vesting schedules, so the only way to support them at the moment is to retire some of the existing pools and create new ones with the required schedules, which in turn requires enabling new nodes to join the validator set, which in turn requires the admin privileges currently granted by pbo_delegation_pool.move to be greatly restricted (since members of the public who have accumulated enough SUPRA will be able to create their own validators and must not be allowed to abuse any delegators that they manage to recruit). However, if we could update the schedules of pools without principle stakeholders (i.e. stakeholders with stake locked by the vesting schedule), which is the case for most of our pools, then we would be able to use our existing delegation pools for the new allocations, which will greatly simplify the integration process.

[isaacdoidge]

There is no way to tell if a delegation pool has no principle stakeholders. If we go ahead with this then we will have to allow the update to occur without this check. This should be fine while the foundation controls all delegation pools as we can manually ensure that we only ever update pools without principle stakeholders, but will need to be removed when the validator set is opened to the public.

[isaacdoidge]

Dr Joshi has clarified that we can use the cumulative_unlocked_fraction to determine whether it is safe to update the vesting schedule. This value will be zero for all delegation pools that do not have locked stakers, as well as for those for which no locked staker has yet tried to unlock their stake.