Fix shard benchmarks
Fix shard benchmarks

at some point, the Inmem field on the engine options became
required, but the benchmarks weren't updated.

also uses filepath everywhere when manipulating file paths.

nooproblem/influxdb,influxdb/influxdb,mark-rushakoff/influxdb,mark-rushakoff/influxdb,li-ang/influxdb,nooproblem/influxdb,linearb/influxdb,mark-rushakoff/influxdb,influxdb/influxdb,nooproblem/influxdb,li-ang/influxdb,nooproblem/influxdb,nooproblem/influxdb,linearb/influxdb,influxdb/influxdb,mark-rushakoff/influxdb,mark-rushakoff/influxdb,influxdb/influxdb,influxdata/influxdb,linearb/influxdb,influxdata/influxdb,influxdb/influxdb,mark-rushakoff/influxdb,linearb/influxdb,linearb/influxdb,influxdb/influxdb