ValidateNode does not check the exact size of the non-leaf nodes

[staheri14]

Problem

In an NMT, non-leaf nodes all have a particular size that is 2*namespace size + hash output size.
The ValidateNode should take into account this size and validate nodes' size accordingly. However, this is not the case in the current implementation.

Acceptance Criteria

Incorporate the exact size check to the ValidateNode method together with proper unit tests.