SQL table blob storage

[foxcpp]

Allows easy backups, etc when used together with storage.imapsql.
See #291 (comment)

Depends on #303.

[theduke]

Postgres supports Large Object storage with a custom interface.
That would make this more feasible for non-tiny deployments.

The currently used driver (pq) does not support this.
pgx does though.

[Avamander]

That would make this more feasible for non-tiny deployments.

At first a separate table with a binary data type column would be nice as well. LOS is a bit more involved to keep track of and back up and not everyone might wish to do so.

[theduke]

Agreed, for my own deployment a regular table would probably be just fine as well.