block cmd: allow adding multiple blocks at once

[magik6k]

This makes block put consistent with dag put (Addressing #4721)

This will be a minor breaking change for js, cc @vmx

[Stebalien]

This will be a minor breaking change for js, cc @vmx

cc @alanshaw.

We could also have two cases: 1 return value and multiple return values. However, I'd rather do it this way.

[Stebalien]

This doesn't appear to break interop. Merging.