Blockstore: Finalize but still readable

[rvagg]

@MichaelMure would like to be able to Finalize() a CARv2 blockstore, so safely flush it to disk along with the index, but still be able to read from it.

This is reasonable and doable, but it means we either have to introduce new APIs or break the Finalize() contract; we probably shouldn't do that.

Finalize() calling ReadOnly#Close() is the key problem here, but if we had a separate ReadWrite#Close() then we could avoid that, but we'd need a separate Finalize(). @MichaelMure has suggested FinalizeReadOnly(). An alternative API he's suggested is for that call to return a closer but that's a bit of a departure from the imperative style here so I'm 👎 on that one. Finalize() can then be a combination of FinalizeReadOnly() and Close() and the functionality provided by ReadOnly can still remain intact and the PutMany check of closed could change to a check on a new finalized property.

@masih @willscott any objections to allowing @MichaelMure to proceed with those changes, or do you have better suggestions, or even objections?

[MichaelMure]

See #376