Use buffered writer when writing tombstones
Use buffered writer when writing tombstones

When deleting many series, the many small writes flood the disks
and consume a lot of CPU time.

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