Migrate to async/await

[fwcd]

With the release of Swift 5.5, concurrency and async/await are now part of the language and it would be great if we could integrate it into D2. Due to the size of the project, a gradual migration would be ideal.

Progress

• Begin migrating to Swift Concurrency by introducing async/await at the top-level #156
• Replace dispatchMain with a sleep loop #158
• Support async in Command, MessageHandler and other core protocols #157
• Make Sink methods and a number of handlers async #160
• Migrate command notification methods to async and clean up Sink #162
• Migrate Receiver and D2Receiver to async #164

To do

• Add basic async/await support at the top-level and in core protocols (e.g. Command)
• Add convenience wrappers around Promise and HTTPRequest in swift-utils
• Gradually migrate commands to async/await