[backup] default chunk size: 128M
[backup] default chunk size: 128M

So when we do concurrent downloading of chunks to memory, they won't take too much of it.
It's possible to use more sophisticated streaming and buffering, but this is straightforward.

Closes: #6699

aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core