Race condition updating column_types.json

[dogversioning]

If a lot of file changes come in at once (not likely in practice, but common if we're debugging/rerunning merges by copying study data around), multiple simultaneous read/write actions to the column types metadata will result in changes being overwritten.

This probably needs to be fixed by piping these message to a queue and processing them in order, or by moving from simple json storage to (more expensive) db storage.