Multithreaded CSV writes

Well, it gives me the same performance as doing it sequentially. I want to write these files as quickly as I could…

julia> @time write_async(dfs)
 51.096555 seconds (806.22 M allocations: 20.268 GiB, 6.99% gc time)

julia> @time write_async(dfs)
 50.875884 seconds (806.19 M allocations: 20.266 GiB, 6.40% gc time)

julia> @time write_async(dfs)
 51.142538 seconds (806.19 M allocations: 20.266 GiB, 6.36% gc time)