Fix concurrent access to result map when merging coverage

[ssams]

resolves #1424

Note that an alternative implementation could be made using p-reduce which might be slightly more efficient as it could start processing whilst files are still in the process of being loaded. However, this would require switching to ESM ... hence I decided on a simpler approach without introducing a new dependency for now (it also would not support limit concurrency at the moment, sindresorhus/p-reduce#4, thus requiring another dependency to p-limit).