Fix race in findGenerations
Fix race in findGenerations

The file store stats slice is re-used which causes the race below:

WARNING: DATA RACE
Write at 0x00c42007e140 by goroutine 43:
  github.com/influxdata/influxdb/tsdb/engine/tsm1.(*FileStore).Stats()
      /Users/jason/go/src/github.com/influxdata/influxdb/tsdb/engine/tsm1/file_store.go:511 +0x22e
  github.com/influxdata/influxdb/tsdb/engine/tsm1.(*DefaultPlanner).findGenerations()
      /Users/jason/go/src/github.com/influxdata/influxdb/tsdb/engine/tsm1/compact.go:461 +0x6f
  github.com/influxdata/influxdb/tsdb/engine/tsm1.(*DefaultPlanner).PlanLevel()

Previous read at 0x00c42007e140 by goroutine 40:
  github.com/influxdata/influxdb/tsdb/engine/tsm1.(*DefaultPlanner).findGenerations()
      /Users/jason/go/src/github.com/influxdata/influxdb/tsdb/engine/tsm1/compact.go:463 +0x13d
  github.com/influxdata/influxdb/tsdb/engine/tsm1.(*DefaultPlanner).PlanOptimize()

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