Fixed DebuggerDisplay labels following recent refactoring
Nicer ToString() for logged commands
Fixed db options setters to extend IFdbDatabase interface, instead of FdbDatabase class
Fixed tr.GetRangeAsync(..) to force iteration to 1, if run with default options

- Iterator streaming mode will fail with invalid_client_opeartion if iteration is equal to 0
- It is ignored in all other streaming modes
Made the ctor of FdbQueryMergeExpression 'protected' instead of 'protected internal'