Decouple chain-sync client and block-fetch

[agaffney]

We currently call block-fetch directly in the chain-sync client RollForward handler. Instead, we should generate an event, and the ledger can decide when/if to fetch the block content. We'll need to provide a way for the ledger to ask for a range of blocks, such as a callback function that the consumer can define. This may need to include a connection ID(s) to ensure we ask a node that has already sent us the block headers