parallel multi-keyring design thoughts

Collecting some thoughts here for the parallel multi-keyring whenever that happens.

- how do you specify the generator?
- My inclination is that parallel multi-keyrings MUST NOT generate. The reason for this is because you would always need to generate before you can fan out. The right way to identify the generator keyring would be to build a multi-keyring with that keyring as the generator and a parallel multi-keyring as a child keyring.
- Controls that we will need to consider include:
- parallelism : how many child keyrings will operate concurrently?
- ordering/grouping/priority : should there be any requirements about one group of child keyrings finishing before another group start?
- My inclination is that if you want this kind of grouping behavior you should get that by combining multiple parallel multi-keyrings as children of a multi-keyring.

No files, tests, or entry points are identified in the issue. Start by reviewing the existing multi-keyring specification and the two comments, then define the parallel multi-keyring behavior and its generator, parallelism, and ordering rules; done means the design questions have an agreed specification.

Written by the indexing model from the issue text.