Do not reset batch ids & redownload out of range batches (#1528)

The changes are somewhat simple but should solve two issues:
- When quickly changing between chains once and a second time back again, batchIds would collide and cause havoc. 
- If we got an out of range response from a peer, sync would remain in syncing but without advancing

Changes:
- remove the batch id. Identify each batch (inside a chain) by its starting epoch. Target epochs for downloading and processing now advance by EPOCHS_PER_BATCH
- for the same reason, move the "to_be_downloaded_id" to be an epoch
- remove a sneaky line that dropped an out of range batch without downloading it
- bonus: put the chain_id in the log given to the chain. This is why explicitly logging the chain_id is removed

beacon_node/network/src/sync/range_sync/mod.rs

@@ -8,6 +8,5 @@ mod range;
 mod sync_type;
 
 pub use batch::Batch;
-pub use batch::BatchId;
 pub use chain::{ChainId, EPOCHS_PER_BATCH};
 pub use range::RangeSync;