Refactor handle_get_next_bandwidth() to use connection_dir_buf_add()

After legacy/trac#21377 (moved) merges, we want to refactor handle_get_next_bandwidth() to use connection_dir_buf_add().

If we backport this refactor to 0.4.0 or earlier, we'll also need legacy/trac#29896 (moved) for the connection_dir_buf_add() backport.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information