Warmup of loading data does not yield correctly

Currently we currently do not yield correctly during warmup (added by MB-47267) after we've spent 10ms loading data from disk. MB-47267 was originally created to help ensure that we warmup buckets more fairly.

This bug is caused by us not taking a reference to the callback in ScanContext which can be seen here (added in commit 423ad543ece154d405de06309601a3bc77f8f399). This commit was made as apart of the original change for MB-47267 so it means that the code has never worked as fully intended.

To fix this we should change auto kvCallback = to auto& kvCallback =.

This doesn't affect 6.6.4 or later as we never made a back port for 6.6.4 even though MB-48091 was created to do this.