Add configurable cache storage adapter

[wschurman]

Similar to #16, though allows user configuration of the storage adapter (still uses localStorage by default). Should be a straightforward rebase with #16 should we merge that below or on top of this.

Making this configurable allows for use in environments without localStorage.

react-native example:

import {
  AsyncStorage,
} from 'react-native';

export default class AsyncStorageCacheStorageAdapter {
  getItem(key: string, callback: (error: any, value: any) => void): void {
    AsyncStorage.getItem(key).then((val) => {
      callback(null, val);
    });
  }

  setItem(key: string, data: string): void {
    AsyncStorage.setItem(key, data);
  }

  removeItem(key: string): void {
    AsyncStorage.removeItem(key);
  }
}

Additional changes:

• Pull in fix to root call map keys: hudl@6eeba15
• Update relay-starter-kit copy (similar to Use localForage #16)
• Remove npm-debug.log and add to gitignore (similar to Use localForage #16)

Definitely open for comments and additional suggestions on the feature.

[brad-decker]

Hey @wschurman - thanks for the PR. I'm gonna review this but i'll need to consult with @aweary who has a great deal more experience with this repo.

[wschurman]

Upon further inspection and use in our app, it looks like having these async writes can cause race conditions when writing to a single storage key. I'm going to do a bit more investigation into distributing the records into their own cache keys or implement a reader-writer lock on the cache to keep it consistent.

I think we can close this in the meantime. I put up another PR if the solution is general enough for inclusion in this repo.