Fixes #2748 by filling the streaming buffers fully

[rbtcollins]

Diagnosis by Daniel. Code by Robert.

[kinnison]

If we're making this change can we also add drop(contents); before the chunk_complete_callback(len); in diskio::write_file_incremental() (mod.rs)?

[rbtcollins]

If we're making this change can we also add drop(contents); before the chunk_complete_callback(len); in diskio::write_file_incremental() (mod.rs)?

Sure. Though as discussed, don't think its material.

[kinnison]

Sure. Though as discussed, don't think its material.

It dropped the max-rss by not-quite-a-chunk in my basic testing