Rework internal 'once' commands

[mpeck]

All commands will become multi commands. That is, they will be run once for each result from the previous stage of the pipeline. Commands will depend on the new memory service to accumulate any state.

Commands to be reworked:

• table
• sort
• unique
• sleep

parent #399
depends on:

• Add additional invocation data to cog_env #618
• Add additional arg for stdin to embedded commands #619

[vanstee]

Now that we have COG_STAGE_POS, COG_SERVICE_TOKEN and COG_INVOCATION_ID we can accumulate state in a unique id for each new item using the memory service, and then on the last item we can retrieve the state, perform an operation on it and return it. I'll start with sort since we've already put together a similar example written in ruby https://gist.github.com/vanstee/a4b1372d3acc35d565c3d82be0f77615 Although, just to be clear, we'll still be writing these in Elixir with GenCommand for now.