Add a flux command for waiting a specific time

[fsteeg]

Use case is a workflow where we use open-http to POST records to an API. When working with large input data, we'd like to wait a short amount of time between each post to avoid creating too much load on the server. While this could be done with an additional option for open-http, perhaps a generic wait command, with the option to set the time would be the best approach?

[blackwinter]

Does wait imply waiting for something? (Like wait-for-inputs for instance.)

Maybe sleep would be better? (Could also be a Fix function; see Catmandu.)