streaming large bodies

[arve0]

Partially read body from database, buffer chunks and convert chunks to a stream.

References:

• https://www.sqlite.org/c3ref/blob_read.html
• https://docs.rs/rusqlite/*/rusqlite/blob/index.html
• https://docs.rs/actix-web/1/actix_web/body/struct.BodyStream.html