Optimize performance of sequential DZKPs

[andyleiserson]

The change in #1350 to use record IDs to index proof batches caused a modest performance regression.

	Before	After
25M	3 hr 4 min	3 hr 16 min
2M	15 min 16 s	17 min 38 s

Comparing the shorter runs, the difference appears to be in the share conversion step. I suspect it has to do with the way that OrderingSender sequences communications (record n+1 is not accepted into the channel until after record n) somehow reducing the amount of validation work that can be done in parallel, but I haven't completely worked out how.

Originally posted by @andyleiserson in #1350 (comment)