Joins an existing pool from another process. Claims a free submitter
slot with its own injection ring and result-slot subrange. The name of
the pool travels out of band: it is mizu_pool_status(p)$name on the
creator.
Examples
p <- mizu_pool()
name <- mizu_pool_status(p)[["name"]]
# `name` travels out of band to the joining process, which runs:
# pa <- mizu_pool_attach(name)
# mizu_collect(mizu_submit(pa, runif(3)))
mizu_pool_stop(p)