insert cached child at the front of a chain of parent lookups

[realbigsean]

Issue Addressed

I induced some parent lookups on devnet 8 and I observed this log on almost every response:

Sep 25 19:41:18.582 DEBG Invalid block received outcome: NonLinearParentRoots, msg: peer sent invalid block

I'm pretty sure this is happening because we append the "cached child" block (the block that triggers the chain of parent requests) to the end of the chain of blocks here: https://github.com/sigp/lighthouse/blob/deneb-free-blobs/beacon_node/network/src/sync/block_lookups/mod.rs#L1196

This comment seems to suggest blocks here should be ordered from highest slot to lowest slot (assuming "highest" and "lowest" here refer to slot). The fact that blocks are reversed here, and later used in order of ascending slot in filter_chain_segment also backs this up.

lighthouse/beacon_node/network/src/network_beacon_processor/sync_methods.rs

Line 403 in 42b34db

// parent blocks are ordered from highest slot to lowest, so we need to process in

[pawanjay176]

Nice catch.
We could consider using a VecDeque to avoid O(n) inserts in Vec though.

[realbigsean]

good idea!