feat: Allow unequal distribution of stake among validators

[fastfadingviolets]

Problem

Currently, all validators start up with the same amount of stake. It's possible to modify what this stake is via ModifyGenesisAmounts in the chain configuration, but that still assumes a single set of genesis and self-delegation amounts, with no way to set it per-validator. This makes it hard to reproduce certain bugs that happen when there's an unequal distribution of stake; one such bug was found on gaia v15.1.0-rc0, for instance.

Proposal

ModifyGenesisAmounts should take a validator index; the returned amounts will be used for that specific validator.

If this sounds like a good idea, I can put up a PR.