Naming and terminology review

[alastairs]

• [x] Ensure all async methods have an Async suffix (#91)
• [x] Define overrides for all async methods accepting a CancellationToken (#92, #96, #97)
• [x] Review error messages for clarity and correctness (#93)
• [x] Ensure no ReSharper warnings or errors (#95)
• [x] Ensure all public methods check for null/empty values (#94)
• [x] Ensure names are consistent across the solution (#98)