Hammer deduplication needs for RAM size

[Venkatesh Srinivas]

I deduped a dataset that from ~600G -> 396G on a system with 256MB of
physical RAM and a 32GB swapcache. Peak Virt size of 'hammer dedup'
was in the 700MB range. double_buffer was on. Performance was pretty
reasonable and the system was plenty usable the whole time. Don't
remember how long it took, though.

-- vs