KeyValueStore Single Callback for Multiple Concurrent Gets

[rdev34]

I ran into the issue where I needed to grab 3 KeyValueStore values at once and saw two ways to structure my code:

1. Nest the requests so that second request is inside callback of first, etc
2. Do all requests concurrently and monitor status of requests until all are complete.

Option 1) is favored because of the low complexity to achieve a block of code that will handle all 3 values, but results in disgusting nesting of KVS requests.
Option 2) has no nesting, but is too complex and verbose to leverage easily because of the need to spin up a thread to wait for the requests.

We need a KeyValueStore Get solution that uses Option 2) under the hood. It runs all the requests concurrently (or aggregates to 1 request), and returns all the results in a single callback.

[rdev34]

Done, KeyValueStore:Get now supports a table of values like:

KeyValueStore:Get({"Key1", "Key2"}, function(values) 
    print(values["Key1"])
    print(values["Key2"])
end)

[rdev34]

Uses KeyValueStore:Get internally to take advantage of caching