Async methods do not support cancellation

[swiftMessenger]

The Certes library makes extensive use of async methods (eg: account registration, order placement and retrieval, authorization retrieval, challenge retrieval, validating challenges, etc...). None of these async methods accept cancellation tokens.

Async methods - particularly those involving network requests - should support cancellation tokens to accomodate scenarios where the calling method needs to terminate execution early (eg: from a system timeout or shutdown request).

This may relatively easily be achieved by adding an overload for async methods to accept a CancellationToken as the final argument (eg: in AcmeContext: public async Task<IAccountContext> Account(CancellationToken cancellationToken = default(CancellationToken))). All async HttpClient operations already support this token so it should mostly be a matter of passing the CancellationToken through to all async methods.