Refuse getting sent the same block when it's already on disk

Description

There is nothing preventing being sent the same block multiple times. As the blocks are verified, a block always stays the same, meaning there is no reason to accept a send request of the same block multiple times (this is a waste of bandwidth)

Solution

In the protocol of send_block_to there is a ExchangeCode enum. Add a code that is AlreadyStored for example. We can't just use RejectBlockSend because that would result in an error, but the block already stored is actually the result we want, so it should result in an Ok.

On the sender side, in handle_send_block_exchange_sender_side_inner in the first answer simply match on the new AlreadyStored variant and return a Ok

On the receiver side, check if the block is already on disk before checking if enough room is available for the block.