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

Add docs for host delegation

well-known 3b: port shoud be included in the Host header

Allow skipping user check in upload_controller.StoreDirect

Fixes https://github.com/turt2live/matrix-media-repo/issues/188

And then stripped from the serverName when checking the cert

Put IPv6 between brackets since we happen the port

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

To prevent accidents like what happened in c30170ff

They're already gzipped going in, just need to be unzipped coming out.

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

Also use c.size instead of usedBytes for accuracy inside the lock

We will clear it ourselves.