Avoid an AwaitUtxo race condition when switching to the non-finalized state

[teor2345]

Motivation

Currently, Zebra ignores any UTXOs in FinalizedState.queued_by_prev_hash, because it is only used during checkpoint verification.

This creates a rare race condition between concurrent:

• pending blocks in the last few checkpoints, and
• the first few blocks doing full validation using UTXOs.

But it doesn't seem to happen much in practice. If it did, some blocks would temporarily fail full validation, until they get retried after the checkpointed blocks are committed.

Suggested Solution

• Make QueuedBlocks generic, and use it for StateService.queued_finalized_blocks
• Check StateService.queued_finalized_blocks in AwaitUtxo
• Do a full sync to check that it's not too slow

Optional:

• Only do the check when we're close to the last checkpoint configured for zebra_consensus::CheckpointVerifier, to avoid cloning a bunch of unused UTXOs. The last checkpoint depends on the checkpoint_sync config.

[teor2345]

I don't think we're going to do this as part of #4937, it's not a release blocker.