fix race condition in go thread pool
fix race condition in go thread pool

raintank/metrictank,raintank/metrictank,replay/metrictank,raintank/raintank-metric,raintank/metrictank,replay/metrictank,replay/metrictank,raintank/metrictank,replay/metrictank,raintank/raintank-metric,raintank/raintank-metric