Connected client and Async

[snej]

Infrastructure:

• Result<T> holds either a T or a C4Error. Similar to types found in Swift, Kotlin, Rust, ...
• Async<T> is a promise/future of a T. (Actually of a Result<T>, since it can fail.) Integrates with Actors.

Connected Client:

• ConnectedClient opens a WebSocket (just like Replicator) and uses the BLIP replicator API to make CRUD requests and observe changes. Results are returned as Async values.
• This required a few additions to the replicator API, so I've extended Pusher and Puller to support them so we can run unit tests.

[snej]

Jenkins iOS build is failing with clang: error: unknown argument: '-fsanitize-ignorelist=/Users/mobile/..../couchbase-lite-core/Xcode/sanitizer-ignore-list.txt'

I'm guessing the Xcode on the build machine has a version of Clang that's old enough not to understand this flag.

[pasin]

@snej Is this ready to review?