take care of closing storage

[tomasciccola]

this library instances a bunch of CoreIndexStreams, each with a storage (random-access-storage) instance. Every CoreIndexStream gets added to the MultiCoreIndexStream.
Options to be able to close every storage instance could be:

1. Expose storage from CoreIndexStream. On MultiCoreIndexStream on the [kdestroyPromise]() method (or a different close method?), iterate over this.#streams, access each instance of storage and close it
2. On MultiCoreIndexer class, keep track of each storage instance (adding them to a list or smth - would I be making copies or are they just references?). This would probably happen on the static method defaultStorage