Tuples: support for bool? and optimized support for int32 and uint32
Changed FdbQueryMergeIterator so that source type is generic

- simplifies intersection between simple queries like index lookups
Fixed FdbSimpleIndex and added unit test

- added ValueComparer and IndexNullValues properties
- Added Lookup() that return an async iterator, and LookupAsync() that executes the iterator
- Allows joining two (or more index) to simulate quries like "SELECT id WHERE cond1 AND cond2" if cond1 and cond2 are supported by two different indexes
Fixed infinite loop in MergeSort if no Limit specified