update the spec without affecting incoming metrics processing

[FrankChen021]

when updating a spec through API, the input source will be unloaded first and then loaded again to take effect. this would cause incoming metrics fail to process between the unloading the loading process because of not found metric handler.

[FrankChen021]

the solution is to (deep) compare if input source has been changed, if not, there's no need to unload the old one