The fallback when blobless quarantine contains a block with all blobs
modifies collection while iterating, potentially asserting if reachable.
Using a second loop to process this situation resolves that.
Add support for using era file for the initial checkpoint block.
This should also avoid an error when the beacon node is restarted
before the backfill process has made any progress (#6059).
`batchVerify`'s precondition is a non-empty signature list:
```nim
if input.len == 0:
# Spec precondition
return false
```
This means that in eras without any blocks (as has happened on Goerli),
calling it leads to era files being reported as invalid.
When initializing from a state that's not aligned to an epoch boundary,
an earlier state is loaded that's epoch aligned, and subsequently topped
up with the missing blocks. `dag.headSyncCommittee` is initialized prior
to topping up the missing blocks, though. If the sync committee changes
while applying the blocks (e.g., a sync committee period boundary hits),
the cached information becomes unlinked from `dag.head`, leading to
valid blocks based on that chain being rejected. To fix this, move cache
initialization after the top up with blocks. This has been observed on
Goerli by initializing from 7919502 and attempting to top up 7920111.
The block gets rejected with an invalid state root on nodes that have
restarted after setting 7920111 as head, while it gets accepted by all
other nodes. Error message is `block: state root verification failed`.
The incorrect initialization behaviour was introduced in #4592, before
which the sync committee cache was initialized after applying blocks.
Iterating peers should only yield peers present in registry, otherwise
`nil` pointers are returned and depending on comparison function it will
break, see #6149.
The `<` function to compare peers was not exported, leading to the same
peer be acquired over and over again until kick. `mixin` doesn't pull it
into `peerCmp` without `*` export, and with the export no mixin needed.
