code: function() {
          const deviceKeys = getKeysWithPrefix(this.store, keyEndToEndSessions(''));
          const results = {};
          for (const k of deviceKeys) {
              const unprefixedKey = k.substr(keyEndToEndSessions('').length);
              results[unprefixedKey] = getJsonItem(this.store, k);
          }
          return results;
      }
docs: Retrieve all end-to-end sessions between the logged-in user and other
  devices.
  @return {object} A map of {deviceKey -> {sessionId -> session pickle}}