Update speed: Summarize documents/legislation/meetings in parallel when possible

[davepeck]

Currently we summarize in sequence.

But there's no fundamental reason we can't summarize multiple documents in parallel; once those are done, move on to legislations; once done, on to meetings.

This task asks me to improve the implementation of the ./engage legistar summarize ... command hierarchy to support this. It might require a decent amount of re-architecting, but if we want new summaries to appear on our website as quickly as possible it seems necessary.