SlotMeta.last_index may be unknown and current code is using u64::MAX to
indicate that:
https://github.com/solana-labs/solana/blob/6c108c8fc/ledger/src/blockstore_meta.rs#L169-L174
This lacks type-safety and can introduce bugs if not always checked for
Several instances of slot_meta.last_index + 1 are also subject to
overflow.
This commit updates the type to Option<u64>. Backward compatibility is
maintained by customizing serde serialize/deserialize implementations.
