Disabling MsgAddVestingOptions message

[lumtis]

MsgAddVestingOptions allows the coordinator to provide shares to an account for a mainnet where the reflected balance will have vesting option.

However, with special allocation, we may have a more generic way to allow coordinator to create vesting options: #767
With GenesisDistribution allocation, the coordinator provides shares allocation for genesis distribution. Accounts are defined in a custom genesis, in this custom genesis, the accounts can acquire vesting options

I would consider disabling MsgAddVestingOptions for the following reason:

• Special allocation is more flexible, we only support Delayed Vesting options in MsgAddVestingOptions while any kind of vesting option like PeriodicVesting can already be provided through special allocation
• Supporting this message adds a burden to the genesis generation because we would need to handle the case where a vesting option is defined for an account in the custom genesis with special allocation and another vesting option is defined with MsgAddVestingOptions => how to combine both vesting options during genesis generation?

I would not remove entirely the feature from the codebase because:

• In the future we might consider implementing Vesting Vouchers, vouchers reflecting shares with vesting options. At this point it would make sense to address the issue above for the client with vesting options defined several times