Override hashCode and equals for MEdge and MVertex classes

I faced a bug while storing MEdge and MVertex in a HashMap. Under certain circumstances, I was not able to retrieve them. To reproduce the issue, I found no other way than create and loop over a large amount of elements (~10k), probably because instances was cached by MapDB. Related unit tests are provided in the patch.

PS1: The Property Graph Model Test Suite does not make use of `Elements` implementation `hashCode()` and `equals()` methods, instead they rely on the `ElementHelper.areEquals()` static method. I don't know why?

PS2: unused imports in existing test classes was cleaned (using Intellij auto-formatter :)