code: function idb() {
    return global.forceIndexedDB
        || global.indexedDB
        || global.webkitIndexedDB
        || global.mozIndexedDB
        || global.msIndexedDB
        || global.shimIndexedDB
  }
docs: Get globally available IDBFactory instance.
  - it uses `global`, so it can work in any env.
  - it tries to use `global.forceIndexedDB` first,
  so you can rewrite `global.indexedDB` with polyfill
  https://bugs.webkit.org/show_bug.cgi?id=137034
  - it fallbacks to all possibly available implementations
  https://github.com/axemclion/IndexedDBShim#ios
  - function allows to have dynamic link,
  which can be changed after module's initial exectution
  
  @return {IDBFactory}