Instead of buffering all the export files into memory (which could be hundreds of gb) we buffer them in one at a time and hope that they don't contain extra information.

On a 171mb export, this change used 45mb of memory at most. Previously the task would have required about 1gb. For larger (200gb+ exports) this is obviously better.

Fixes https://github.com/turt2live/matrix-media-repo/issues/100

Fixes https://github.com/turt2live/matrix-media-repo/issues/262

Fixes https://github.com/turt2live/matrix-media-repo/issues/146

Fixes https://github.com/turt2live/matrix-media-repo/issues/218

Fixes https://github.com/turt2live/matrix-media-repo/issues/233

Fixes https://github.com/turt2live/matrix-media-repo/issues/49

Fixes https://github.com/turt2live/matrix-media-repo/issues/268

Fixes https://github.com/turt2live/matrix-media-repo/issues/269

Fixes https://github.com/turt2live/matrix-media-repo/issues/265

Also fixes an issue with apng thumbnails being cropped in multiplying ways.

It wasn't being used anyways