Signed-off-by: Roman <bapstroman@gmail.com>

Signed-off-by: Roman <bapstroman@gmail.com>

- fsync can be a long blocking operation, so we need to make sure the main
  logger thread does not block during this time, when it tries to aquire
  the mutex
- fixes calling fsync on an invalid file descriptor

 - only used in test routines

 - only SENSOR_POLLRATE_DEFAULT is needed

 - only used in self tests