Ensure file contents flushed to disk before returning HTTP status

[leondutoit]

The combination of async writes and random storage errors can result in data loss. One idea to mitigate this without too much of a performance hit would be to stat the file after closing the file descriptor in the implementations of put and patch, and then to compare the bytes to the amount of bytes received (written according to the application), and then if the byte counts do not match, to return 500. Clients would then need to retry the request until successful.