Add support for submessages/callback

[ethanfrey]

This is based on CosmWasm issue CosmWasm/cosmwasm#691 and PR CosmWasm/cosmwasm#796.

Note: this is intended for v0.16.0. It is also a large change and we could wait until v0.17.0 to implement it and for now just throw "unimplemented" error if any contract returns subcall. Let's evaluate this one well.

We need to add a new subcall_response entry point similar to #420, however, this entry point is only used internally in the keeper and not exported to any other module.

We also need to handle submessages in the Response. These have the following semantics:

• Store the parent context
• Wrap the current context/storage in a new context (with CacheWrap())
• If a gas limit is set, and limit is less than current gas used, create a new gas meter with smaller limit and update context
• Call the message in this new context (just as we handle messages)
• If we set a custom gas limit:
  • If we get an OutOfGas error, charge the parent context the full gas allowance of this subcommand and return error
  • Otherwise, charge the parent context the actual gas used
  • If either cause an OutOfGas on the parent context, we don't catch that and allow it to abort the full connection
• Check result and handle storage:
  • If success, Commit the CacheWrap to the parent storage
  • If error, "rollback" (drop) the CacheWrap
• Return the result to the calling contract via new subcall_response entry point
  • If error, return a string error result
  • If success, return all events and data set in the submessage
    • Also add these events to the parent transaction (to return to original caller, just as normal messages)
    • We do not charge the instantiation cost (40.000 gas) on this subcall_response as the parent is assumed to be cached.
• Process subcall_response result just like any other result
  • note that subcall_response from message 1 is processed before message 2
  • note all submessages (and their responses) are processed before all normal messages

You can test this with the reflect contract, with has a new HandleMsg::ReflectSubCall variant to test this, and just stores the subcall_response in the storage for later queries (and test assertions)