Reduce allocations when creating KeyCursors
Reduce allocations when creating KeyCursors

The refs map was to increment the file references one time each.
It doesn't hurt to increment them multiple times though.

We also do not need to copy the files slice as we are accessing it
under a read lock so it can't be changed.

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