Fixes https://github.com/turt2live/matrix-media-repo/issues/267

We make our own ReadCloser+ReadSeeker implementation because a NopCloser actually destroys the seeker implementation.

This is a step towards video processing, maybe.

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

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

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.

This removes a select across the whole table by ensuring we generate a media ID which is highly unlikely to conflict with the database.

This should help memory some more. See https://github.com/turt2live/matrix-media-repo/issues/237

Theory is that by dumping the streams we'll relieve any bytes held on by the S3 libraries. 

Might reduce memory usage over time

Fixes https://github.com/turt2live/matrix-media-repo/issues/223

Fixes https://github.com/turt2live/matrix-media-repo/issues/209

Part of https://github.com/turt2live/matrix-media-repo/issues/209

Part of https://github.com/turt2live/matrix-media-repo/issues/209

Part of https://github.com/turt2live/matrix-media-repo/issues/209

Fixes https://github.com/turt2live/matrix-media-repo/issues/183

3% corruption rate