Operator: Append

[carmeleve]

Link to Rx.NET implementation

[bartdesmet]

Note for implementers, this is a little tricky because the operator goes through some state transitions in its subscription tree. However, one can have a look at StartWith to understand the complexity (and to see an example of how we do fine-grained OnNext calls to drain the array, allowing for scheduler responsiveness when attempting to perform a pause operation).