KeyChain architecture improvements

[Avvrik]

As of the last KeyChain presentation, it has become clear that we have dragged a lot of rudimentary elements of architecture from Array's KeyChain. Those elements make no sense and make our API look unprofessional. Hence, they need to be removed and the whole architecture should be given a new thought and restructured.

What needs to be done:

1. Rethink the whole architecture: what should be done to make API more precise, to the point, and efficient.

2. Change the names of some of the commands, so that they reflect their purpose better (e.g. sign hex command)

3. Remove, change or add some of the parameters (e.g. remove chain_id parameter)

This should result in a better, more succint and professional architecture and API.