## Issue Addressed
NA
## Proposed Changes
There are four new conditions introduced in v0.12.3:
1. _[REJECT]_ The attestation's epoch matches its target -- i.e. `attestation.data.target.epoch ==
compute_epoch_at_slot(attestation.data.slot)`
1. _[REJECT]_ The attestation's target block is an ancestor of the block named in the LMD vote -- i.e.
`get_ancestor(store, attestation.data.beacon_block_root, compute_start_slot_at_epoch(attestation.data.target.epoch)) == attestation.data.target.root`
1. _[REJECT]_ The committee index is within the expected range -- i.e. `data.index < get_committee_count_per_slot(state, data.target.epoch)`.
1. _[REJECT]_ The number of aggregation bits matches the committee size -- i.e.
`len(attestation.aggregation_bits) == len(get_beacon_committee(state, data.slot, data.index))`.
This PR implements new logic to suit (1) and (2). Tests are added for (3) and (4), although they were already implicitly enforced.
## Additional Info
- There's a bit of edge-case with target root verification that I raised here: https://github.com/ethereum/eth2.0-specs/pull/2001#issuecomment-699246659
- I've had to add an `--ignore` to `cargo audit` to get CI to pass. See https://github.com/sigp/lighthouse/issues/1669
## Issue Addressed
NA
## Proposed Changes
Shift practically all HTTP endpoint handlers to the blocking executor (some very light tasks are left on the core executor).
## Additional Info
This PR covers the `rest_api` which will soon be refactored to suit the standard API. As such, I've cut a few corners and left some existing issues open in this patch. What I have done here should leave the API in state that is not necessary *exactly* the same, but good enough for us to run validators with. Specifically, the number of blocking workers that can be spawned is unbounded and I have not implemented a queue; this will need to be fixed when we implement the standard API.
## Issue Addressed
- Resolves#1451
## Proposed Changes
- Restricts the `contains_block` and `contains_block` so they only indicate a block is present if it descends from the finalized root. This helps to ensure that fork choice never points to a block that has been pruned from the database.
- Resolves#1451
- Before importing a block, double-check that its parent is known and a descendant of the finalized root.
- Split a big, monolithic block verification test into smaller tests.
## Additional Notes
I suspect there would be a craftier way to do the `is_descendant_of_finalized` check, but we're a bit tight on time now and we can optimize later if it starts showing in benches.
## TODO
- [x] Tests
