* Ignore subset aggregates
When aggregates are propagated through the network, it is often the case
that a better aggregate has already been seen - in particular, this
happens when an aggregator has not been able to include itself in the
mesh and therefore publishes an aggregate with only its own
attestations.
This new ignore rule allows dropping all aggregates that are
(non-strict) subsets of aggregates that have already been seen on the
network. In particular, it does not mandate dropping aggregates where a
union of previous aggregates would cause it to become a subset).
The logic for allowing this is based on the premise that any aggregate
that has already been seen by a peer will also have been seen by its
neighbours - a subset aggregate (strict or not) brings no new value to
the aggregation algorithm, except in the extreme edge case where you
could combine several such sparse aggregates into a single, more dense
"combined" aggregate and thus use less block space.
Further, as a small benefit, computing the `hash_tree_root` of the full
aggregate is generally not done -however, `hash_tree_root(data)` is
already done for other purposes as this is used as index in the beacon
API.
* add subset ignore rule to sync contributions as well
* typo
When a light client updates its `finalized_header` using a forced update
because of the timeout, and the new header was not signed by enough sync
committee participants to pass `get_safety_threshold(store)`, it may
occur that `store.finalized_header.slot > store.optimistic_header.slot`.
This patch ensures that the `optimistic_header` is updated to the latest
`finalized_header` if that happens, so that it always indicates the
latest known and accepted head.
There were a couple instances where a division was used on an epoch
to derive the corresponding sync committee period instead of calling the
`compute_sync_committee_period` function.
These instances were changed to also use the function.
In the light client docs a mentioning of a function trigger is lacking
the `genesis_validators_root` argument. This patch adds that argument
to the documentation to match the real function signature. It also
slightly improves the grammar.
This renames the `sync_committee_aggregate` field of `LightClientUpdate`
to `sync_aggregate` for consistency with the terminology in the rest of
the spec.
1. Replace `header` and `finality_header` with `attested_header` (always the header signed by the committee) and `finailzed_header` (always the header verified by the Merkle branch)
2. Remove `LightClientSnapshot`, fold its fields into `LightClientStore` for simplicity
1. Simplify `valid_updates` to `best_valid_update` so the `LightClientStore` only needs to store O(1) data
2. Track an optimistic head, by looking for the highest-slot header which passes a safety threshold
