Implement "validator_votes" query for valset contract

[alpe]

As discussed on slack the votes on begin_block can be "stored" in memory for the current block only.

Steps:

• Collect votes from RequestBeginBlock.LastCommitInfo in poe/abci.go and keep them in memory.
• implement custom tgrade query for valset

[alpe]

As this touches the internal query API, all validators need to upgrade or it can be state breaking when called

[daniellarita]

@maurolacy will create contract related ticket for this and link it here. Thanks

[maurolacy]

The relevant issue in poe-contracts is https://github.com/confio/poe-contracts/issues/149.