baseapp: Functional options should use setters and seal

[ValarDragon]

Currently, our initialization uses functions on un-exported variables. This means that all possible functions we'd ever want to be defined for initialization must be defined in the baseapp module. Therefore if there is some fancy type of initialization I want as an end user on the variables set in baseapp, I have to mod the sdk. I don't think we should limit what end users can do like that. Note, several of the unexported functions do have this already. E.g. SetTxDecoder, SetAnteHandler. (Albeit not with the proposed seal method)

Instead I propose we have setter variables on each of the unexported variables, and a Baseapp.Seal() method. Once the Baseapp.Seal has been called, setters no longer set. I think this will allow for greater extensibility of baseApp.

[alexanderbez]

We definitely need a Seal method. Can you elaborate on "setter variables"? Maybe give a trivial example?

[ValarDragon]

Woops, I meant to write setter functions there.

[rigelrozanski]

Great design pattern - we can make it crystal clear in the code what the seal does