Lack of concat_map / concat_all

[sshumaker]

Is your feature request related to a problem? Please describe.
We're missing a basic capability - concat_map / concat_all. We're also missing alternatives that would allow it to be easily implemented.

Describe the solution you'd like
Implement concat_map and concat_all. The key is to behave like flat_map with a max concurrency of 1, so items are fully flattened but in sequential order.

Describe alternatives you've considered
Maybe I'm missing something, but even flat_map isn't documented. How are you supposed to handle higher order observables?