online config upgrade changes might be lost if ns_config restarts, leading to inconsistent config (was: server losing connection on newly installed kv+index node (500 from server when GET-ting server groups endpoint))

Manual testable
1. Install 350-1249 on single node linux/windows
2. Setup single bucket, and node w/ kv+query services.

The node keeps losing connection as shown on the screenshot.

and seeing some errors like below on the logs.

Client-side error-report for user undefined on node 'ns_1@127.0.0.1':
User-Agent:Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:35.0) Gecko/20100101 Firefox/35.0
Got unhandled error:
TypeError: can't convert null to object
At:
http://10.6.2.224:8091/js/misc.js line 80 > Function:3:0
Backtrace:
anonymous/<@http://10.6.2.224:8091/js/misc.js line 80 > Function:3:1
renderRawTemplate@http://10.6.2.224:8091/js/misc.js:341:17
renderTemplate@http://10.6.2.224:8091/js/misc.js:359:10
renderStartNewView/<@http://10.6.2.224:8091/js/logs.js:346:9
Cell<.getValue/<@http://10.6.2.224:8091/js/cells.js:368:9
Slave<.nMoreTimes/this.thunk@http://10.6.2.224:8091/js/cells.js:26:7
CallbackSlot<.broadcast/<@http://10.6.2.224:8091/js/cells.js:60:9
_.forEach@http://10.6.2.224:8091/js/underscore.js:76:7
CallbackSlot<.broadcast@http://10.6.2.224:8091/js/cells.js:56:5
notifyWatchers@http://10.6.2.224:8091/js/cells.js:357:9
.completeCellDelay@http://10.6.2.224:8091/js/cells.js:661:7
.completeGeneration@http://10.6.2.224:8091/js/cells.js:647:7
Cell<.tryUpdatingValue@http://10.6.2.224:8091/js/cells.js:426:9
.invokeUpdates@http://10.6.2.224:8091/js/cells.js:630:7
wrap/_super.bugsnag@http://10.6.2.224:8091/js/bugsnag.js:128:23
hijackTimeFunc/</<@http://10.6.2.224:8091/js/bugsnag.js:583:13

Attached logs.