Enhance validation to make it work with internal messages

Some messages can have other messages as fields and these messages should be validated. For example MessageRep has 'params' field which contains messages of specific type. As of now they are validated using external logic, take a look at MessageReq nad CurrentState message handlers at node. This approach is not consistent.

Steps:

1. Make message validator be able to validate internal messages
(it is partially implemented, take a look at MessageField class)

2. Make message validator set parsed values as its fields instead of raw values