Refactoring: Move identifier checks into Context::get{Input, Reference}

The following is an example of the current situation. 
```Kotlin
val client = ctx.getReference<ClientState>()
require(client.id == Identifier(msg.clientId)){"mismatch client"}
```

This is a boring and error-prone code.
The process of getting a state followed by checking the identifier of it should be combined into a single method.