Rewrite / check our equals/hashcode methods

[sbernard31]

Investigating on NPE in LwM2mPath.hashcode() find at #1502.

I decide to add unit tests to check the bug before to fix it.

I found that library to do that job : EqualsVerifier.

Good news : it does a very good job.
Bad news: this is more complicate than expected to implement good custom equals/hashcode method.

We should :

1. Have a look at all custom equals/hashcode methods and add a corresponding test based on EqualsVerifier.
2. Decide some convention about written equals/hashcode in Leshan.
3. Reimplement equals/hashcode if needed.
4. Add documentation about that in : https://github.com/eclipse-leshan/leshan/wiki/Code-&-design-guidelines#code-rules

Let's use this issue to talk about "Decide some convention about written equals/hashcode in Leshan".
If you interested about that you should first read : How to Write an Equality Method in Java

My current opinion :

• we should probably use Objects.hash and Objects.hash

• Use of this 3 a valid behavior :
  1. Use classic getClass() != obj.getClass() + making the class final : this is the simple way but I do not like too much the idea I prefer to let user being able to extend class.
  2. Use an instanceof + make hashcode/equals final : this is simple enough and this allow to extend the class but not to customize equals behavior...
  3. Use the canEqual method, which is a bit unusual but sounds to solve all problems.

  Should we encourage to use 3. in all code base ?