Reduce the max allowed blob size to fit in a 32GiB CAR

[masih]

This number is actually too large to always fit in a single piece.
we need to:

• incorporate bit padding ( actual max CAR Size 32GB * 127/128)
• factor in extra CAR bytes for CIDS + Car Headers + UnixFS intermediate nodes
• I wonder if we just standardize on something safe-ish like 31GB?

Originally posted by @hannahhoward in #26 (comment)

[TorfinnOlsen]

31 is a clean round number that leaves no room for confusion in my opinion. I gather that you lose a little storage this way, but atleast for testing purposes during MVP seems reasonable.

If however we think this is a long term limitation we're going to have to live with, and therefore once used at scale we're sacrificing a great deal of storage I'd say let's think through what a minimal capacity reduction would be.

[masih]

Going with 31Gi. We can increase as needed.