Range scan gets stuck when concurrency or more than 1 IO threads are used

When both concurrency of more than 1 and more than 1 IO threads are used, range scans occasionally get stuck waiting for the next item future (https://github.com/couchbaselabs/couchbase-cxx-client/blob/main/core/scan_result.cxx#L35), after some items have been returned. The vbucket scans all get completed successfully.

From the attached logs (from running the FIT tests with concurrency 10 and 8 IO threads - and some additional logging around the next() function) it can be seen that all scans have been completed and all threads correctly detect that there are no more vbuckets to scan. The first few calls to next() return an item, the last one which happens relatively early on during the scans never returns. This does not always happen with the same test.