CacheMap.get calls should await the promised value

https://github.com/graphql/dataloader/blob/015a94c8db0b4974838a312634c12a2c5eb3d056/src/index.js#L88-L97

Seems like this prevents the use of async calls to centralized caches in it's current implementation since the returned promise will be 'Truthy' even if it resolves with a cache miss (null, undefined).

Wondering if there is a design reason for this, or a PR is welcome ?