Light Syncing Mode

[patrick-ogrady]

Instead of storing the value to the DB, allow node operators to just store the length of the value (minimum needed for block execution) for keys/prefixes they don't care about.

This allows for very lightweight running of nodes and/or just consensus nodes.

This mode should also delete historical blocks (if the rpcchainvm supports this).

We could also change the canonical form of the block to not actually include values. Instead, they could be attached and fetched separately for anyone that wants to validate their correctness/serve data.

[patrick-ogrady]

After #40, we only store the minimum required amount of data to serve block queries. If we want to serve any less, it will require changes in AvalancheGo that allow a node to specify it doesn't have a certain range of blocks.

[patrick-ogrady]

#79 made this much easier to do.