feature: KVStore improvements: JSON in & out, without exceptions

[yogeshbdeshpande]

Original issue reported by @thomas-fossati , as below:

Extracted from veraison/veraison#135:

I am tempted to always have valid JSON in and out of the kvstore. The present exception being the Get interface, which returns a Golang array of JSON objects. It could return a JSON array of all the Added objects instead.

[setrofim]

Is this something that we still actually want? KVStore interface is based on strings, not JSON. Currently Get() returns a slice of strings, not necessarily JSON objects. And, if I understand this correctly, this suggests it should return a single string encoding a JSON array? This would only complicate things for the user, requiring them to always perform the decoding of the array before they can do anything with the value.