INI writes are not atomic

(1) Writes to the INI file are not atomic in couch_config_writer:save_to_file.
This may cause the INI file to be corrupted.

This was likely the cause for:
CBSE-14589

CBSE-14099

Also need to check other places where file:write_file is used. It looks like misc:atomic_write_file is used in many places in ns_server but not all. Need to check whether this is sufficient or atomic writes are necessary in other places.
(2) In addition, in ns_server, we don't check the status of a couch_config:set to confirm it succeeded. It may silently fail and corrupt the INI in the process.

• On 7.2 and above, we at least check the status of couch_config:set when the db/index directories are updated.
  On older releases, we ignore the status of couch_config:set when db/index directories are updated.
• On all releases, we ignore the status of couch_config:set while setting log level.