Speed up drop retention policy
Speed up drop retention policy

Reduce the lock contention on tsdb.Store by taking a short lived
read-lock instead of a long write lock.  Also close shards in parallel
and drop the whole RP dir in bulk instead of each shard dir.

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