Added helpers to create composite tuple comparers (up to three items)

- FdbTupleComparisons.Composite<string, int>() return an IComparer<IFdbTuple> that compares the first element as a string, and the second as an int
- They can compare from the start or from any offset
- ("bbb",) is smaller than ("bbb", 123), but is still larger than ("aaa", 999999)
Merge queries: testing new overrides for Except(...)

- sometimes easier to pass a list of FdbKeyRange and of pairs or key selectors
- added unit test that uses a composite tuple comparer