It would appear that we have been making concurrent accesses to
statsCollector for a long, long time (we expect there to be multiple
calls to `Repair()` at the same time on the same instance of
`SegmentRepairer`, up to `config.MaxRepair`, and before this change
there was no sort of synchronization guarding accesses to the
`statsCollector.stats` map.
Refs: https://github.com/storj/storj/issues/6402
Change-Id: I5bcdd13c88913a8d66f6dd906c9037c588960cc9
