bench: fix loop starvation bug
bench: fix loop starvation bug

Don't keep writing until the write queue fills up. On fast systems (mine), that
never happens - the data is sent out as fast as the benchmark generates it.

urbit/archaeology2,urbit/archaeology2,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso