Memory Store: fixed funkiness in unit tests

- ColaRangeDictionary<T> is broken !
Memory Store: added failing unit test for ColaRangeDictionary<T>

- inserting a non overlapping segment between two segments overwrites the end bound of the first segment
Memory Store: attempted to fix ColaRangeDictionary<T>

- insert in the middle of two ranges, without touching, should not clobber the end of the first range
- plugging a hole with same value as left/right should merge the three ranges into one
- need more tests in corner cases !