Make range sync chain Id sequential

[dapplion]

Issue Addressed

Currently, we set the chain_id of range sync chains to u64(hash(target_root, target_slot)), which results in a long integer.

Jan 27 00:43:27.246 DEBG Batch downloaded, chain: 4223372036854775807, awaiting_batches: 0, batch_state: [p,E,E,E,E], blocks: 0, epoch: 0, service: range_sync

Proposed Changes

Instead, we can use network_context.next_id() as we do for all other sync items and get a unique sequential (not too big) integer as id.

Jan 27 00:43:27.246 DEBG Batch downloaded, chain: 4, awaiting_batches: 0, batch_state: [p,E,E,E,E], blocks: 0, epoch: 0,  service: range_sync

Also, if a specific chain for the same target is retried later, it won't get the same ID so we can more clearly differentiate the logs associated with each attempt.

[AgeManning]

If this helps debugging moving away from a hash-based identification, then i'm all for it.

The changes here seem fine to me.

[jimmygchen]

If we have multiple chains with the same target, this will return the first one.
But this isn't possible I think because we only create a new chain when we cannot find one matching the same target (the None case), so I think this is fine.

[jimmygchen]

LGTM