Fix race in findGenerations
Fix race in findGenerations

It was possible that the findGenerations could get stuck returning
no files even when generations existed on disk.

li-ang/influxdb,nooproblem/influxdb,vladlopes/influxdb,nooproblem/influxdb,influxdb/influxdb,vladlopes/influxdb,runner-mei/influxdb,linearb/influxdb,li-ang/influxdb,linearb/influxdb,mark-rushakoff/influxdb,influxdb/influxdb,influxdb/influxdb,vladlopes/influxdb,influxdb/influxdb,runner-mei/influxdb,benbjohnson/influxdb,influxdata/influxdb,influxdb/influxdb,influxdata/influxdb,vladlopes/influxdb,mark-rushakoff/influxdb,nooproblem/influxdb,archsaber/influxdb,archsaber/influxdb,chenziliang/influxdb,linearb/influxdb,linearb/influxdb,benbjohnson/influxdb,runner-mei/influxdb,mark-rushakoff/influxdb,nooproblem/influxdb,benbjohnson/influxdb,archsaber/influxdb,benbjohnson/influxdb,runner-mei/influxdb,chenziliang/influxdb,influxdb/influxdb,mark-rushakoff/influxdb,mark-rushakoff/influxdb,chenziliang/influxdb,chenziliang/influxdb,linearb/influxdb,nooproblem/influxdb,archsaber/influxdb,mark-rushakoff/influxdb