Fixes https://github.com/turt2live/matrix-media-repo/issues/311

Fixes https://github.com/turt2live/matrix-media-repo/issues/271 at the same time

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

The new parameter `filterUserDuplicates` is set to true
everywhere else except in the import_synapse call. This
ensures direct duplicates from the same user will get
stored in an import from Synapse situation when migrating
to the mediarepo.

Closes #241

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. 

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/187

Fixes https://github.com/turt2live/matrix-media-repo/issues/163

Luckily it would have flagged the upload as quarantined, but still not a good thing to have.

Fixes https://github.com/turt2live/matrix-media-repo/issues/170

There's some other functions which still need some work in order to fully complete the upload process

Another step forwards to having different storage backends. s3 uploads currently not implemented

Fixes https://github.com/turt2live/matrix-media-repo/issues/129

Fixes https://github.com/turt2live/matrix-media-repo/issues/20

**Caution**: This is a major migration that changes how files are tracked within the database. Although tested to ensure things won't go wrong, there is the possibility of something going haywire. It is recommended to run the following database commands before upgrading:

CREATE TABLE media_backup AS TABLE media;
CREATE TABLE thumbnails_backup AS TABLE thumbnails;

In the event you need to roll back, run the following:
DROP TABLE media;
DROP TABLE thumbnails;
CREATE TABLE media AS TABLE media_backup;
CREATE TABLE thumbnails AS TABLE thumbnails_backup;
DELETE FROM gomigrate WHERE migration_id = 7;

This will cause any files uploaded between upgrading and rolling back to be lost.

NOTE: The migration will take a while and runs on the first access of a file. If you have a lot of files, this could take a while.

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Fixes https://github.com/turt2live/matrix-media-repo/issues/106
Step towards https://github.com/turt2live/matrix-media-repo/issues/47

Future support will include deleting media that hasn't been accessed in a while

Part of #58