flat inst

```
     ALL:

    please comment: make flat vs keep nested?

    currently we have nested protobuf object structure with extensions
    https://github.com/barchart/barchart-proto/tree/master/barchart-proto-buf-inst/src/main/resources/protobuf
    like base instrument and then equity, future, etc on top of base.

    pro:
    * nice OO design
    * clear separation of concerns

    con:
    * need to manage protobuf extensions
    * need visitor pattern
    * slower codec

    question: do we make it flat or do we keep it nested?

    Thank you;

    Andrei. 
```