State Proofs: Light client for lists

[ashcherbakov]

Make sure that we can trust a result in cases where multiple results are returned (for example get-all-models).

Options
O1: Do not use Cosmos queriers to return multiple entries. Have an additional index in store <all-items-key>:<list of individual item keys>, so that get_all_xxx can be queried from store (and hence with common state proofs) and returns a list of individual item keys instead of the full item. Individual items cam be queried by get_xxx with the given item key as an input (with a common state proof again).
Note: need to think about pagination support.
O2: Support multi-proof on Cosmos-sdk level (consider contribution).

O2: Multi-Proof support at cosmos-sdk
The current Design: https://github.com/zigbee-alliance/distributed-compliance-ledger/blob/master/docs/design/multiproofs-design.md

Some notes (can be a bit outdated):

1. https://github.com/cosmos/cosmos-sdk/blob/master/client/context/query.go#L102 - light client is used when QueryState is used for querying.
2. if QueryWithData is called (as in all examples such as https://github.com/cosmos/sdk-tutorials/blob/master/nameservice/x/nameservice/client/rest/query.go#L19), then light client is not used and just gets the result from the corresponding business logic (keeper):
   https://github.com/cosmos/sdk-tutorials/blob/master/nameservice/x/nameservice/keeper/keeper.go#L35 or https://github.com/cosmos/sdk-tutorials/blob/master/nameservice/x/nameservice/keeper/keeper.go#L105
   https://github.com/cosmos/cosmos-sdk/blob/master/store/iavl/store.go#L193
   https://github.com/tendermint/iavl/blob/master/immutable_tree.go#L153
3. There is a related issue: Subspace Query does not return Proof cosmos/cosmos-sdk#5241
4. It can be fixed if

• always use QueryStore when querying by key
• when querying a list, call getRangeProof (need to modify cosmos-sdk?)
• https://github.com/tendermint/iavl/blob/master/proof_range.go#L314
• https://github.com/tendermint/iavl/blob/master/proof_test.go#L117