Mutable DTOs can't honour equals semantics

[jimirocks]

The core concept of gooddata-java - mutable dtos is effectively preventing such objects to have valid equals method. It's a question how this issue should be resolved:

❓ make the DTOs immutable and provide copy constrcutors/builders
❓ stay on the current state - the equals is only implemented on objects which has sensible immutable id
❓ something else

[xMort]

I vote for the first option.

[jimirocks]

Let's also unify the equals/hashCode style and write it to guidelines