Breaking Change 4 of 3: Renamed 'IFdbReadTransaction' to 'IFdbReadOnlyTransaction', and cleand up the IFdbXXXTransactional interfaces

- IFdbReadOnlyTransaction only expose methods that can read from the DB, and IFdbReadOnlyTransactional only expose ReadAsync(...) methods
- IFdbTransaction can either write -only, or read-write. IFdbTransactional exposes WriteAsync(...) and ReadWriteAsync(...) methods
- updated a few XML Comments
Folded the IFdbXXXTransactional implementation back into the main FdbDatabase.cs file
FdbTransaction now also implement IFdbTransactional and IFdbReadOnlyTransactional

- Current implementation will call the lambda inline, without any retry logic.
- This emulates the way Transactionals are implemented in the Ruby binding
Exposed the Context property on the IFdbReadOnlyTransaction interface
Cleaned up XML comments (removed "ct")