Chain sync bulk mode causes random disconnects

[agaffney]

We use chain-sync at the beginning to get the available block range and then leave it in the Idle state, which has a (random) timeout. This causes the connection to get randomly closed after the timeout expires if we're still in the middle of receiving blocks via block-fetch.

This probably requires https://github.com/blinklabs-io/gouroboros/issues/597 to fix properly so that we can re-use chain-sync later on the same connection.