Like the other pipeline algorithms in targets, https://github.com/ropensci/targets/blob/main/R/class_crew.R uses polling and exponential backoff to balance latency with CPU usage. nanonext condition variables may help us avoid polling altogether.

This issue depends on wlandau/crew#108, which depends on https://github.com/shikokuchuo/nanonext/discussions/20 and https://github.com/shikokuchuo/nanonext/discussions/21.