Use unexported types for core query servers

[damiannolan]

Summary

Introduce unexported types for core query servers in ibc-core:

• 02-client
• 03-connection
• 04-channel

Limit the api surface of each of the keepers belonging to core subpackages.
Add a private struct which embdeds the Keeper type, and move QueryServer interface methods to this type.

var _ types.QueryServer = (*queryServer)(nil)

type queryServer struct {
    *Keeper
}

Note, that this will have a knock on affect to how we register the query servers in the ibc core module.
As it exists today, the IBCKeeper implements every query servers through core/keeper/grpc_query.go, with a passthrough function for every query handler. This file is unnecessary and could be removed. As a result, we may register query servers like below.

- types.RegisterQueryService(cfg.QueryServer(), am.keeper)
+ clienttypes.RegisterQueryServer(cfg.QueryServer(), clientkeeper.NewQueryServer(am.keeper.ClientKeeper))
+ connectiontypes.RegisterQueryServer(cfg.QueryServer(), connectionkeeper.NewQueryServer(am.keeper.ConnectionKeeper))
+ channeltypes.RegisterQueryServer(cfg.QueryServer(), channelkeeper.NewQueryServer(am.keeper.ChannelKeeper))

Considerations

As mentioned above, the IBCKeeper currently implements every QueryServer interface of each subpackage in ibc core.
The QueryServer interface in core/types, composes each of the subpackage QueryServers into one interface.
This interface is composed by the TestChain in ibctesting, where the IBCKeeper is passed the concrete implementation.

If the grpc_query.go file is removed this will break that abstraction. However, we can explore alternative options when working on this issue. It may be possible to still include some facade pattern struct to provide access to these query handlers.
The QueryServers can still be created on demand by passing the respective subpackage Keeper to the public constructor function. E.g. clientkeeper.NewQueryServer

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