save state: export pending list as array of json strings + fix importing save state to support pending

[ikreymer]

The save state export accidentally exported the pending data as an object, instead of a list of JSON strings, as it is stored in Redis, while import was expecting list of json strings. The getPendingList() function parses the json, but then was re-encoding it for writeStats(). This was likely a mistake. This PR fixes things:

• support loading pending state as both array of objects and array of json strings for backwards compatibility
• save state as array of json strings
• remove json decoding and encoding in getPendingList() and writeStats()

Fixes #568

[edsu]

~~I think this is a problem for queued and failed as well?~~

Nevermind, I see those sections actually expect serialized JSON objects?

[tw4l]

~I think this is a problem for queued and failed as well?~

Nevermind, I see those sections actually expect serialized JSON objects?

I believe these both should be json strings, similar to pending. The failed gets JSON.parse()d prior to being loaded back into Redis. In my testing this is working as expected.