Put IPv6 between brackets since we happen the port

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.

Appears to be still relatively on par with previous results.

Fixes https://github.com/turt2live/matrix-media-repo/issues/232