## Proposed Changes
While investigating an incorrect head + target vote for the epoch boundary block 708544, I noticed that the state advance failed to prime the proposer cache, as per these logs:
```
Mar 09 21:42:47.448 DEBG Subscribing to subnet target_slot: 708544, subnet: Y, service: attestation_service
Mar 09 21:49:08.063 DEBG Advanced head state one slot current_slot: 708543, state_slot: 708544, head_root: 0xaf5e69de09f384ee3b4fb501458b7000c53bb6758a48817894ec3d2b030e3e6f, service: state_advance
Mar 09 21:49:08.063 DEBG Completed state advance initial_slot: 708543, advanced_slot: 708544, head_root: 0xaf5e69de09f384ee3b4fb501458b7000c53bb6758a48817894ec3d2b030e3e6f, service: state_advance
Mar 09 21:49:14.787 DEBG Proposer shuffling cache miss block_slot: 708544, block_root: 0x9b14bf68667ab1d9c35e6fd2c95ff5d609aa9e8cf08e0071988ae4aa00b9f9fe, parent_slot: 708543, parent_root: 0xaf5e69de09f384ee3b4fb501458b7000c53bb6758a48817894ec3d2b030e3e6f, service: beacon
Mar 09 21:49:14.800 DEBG Successfully processed gossip block root: 0x9b14bf68667ab1d9c35e6fd2c95ff5d609aa9e8cf08e0071988ae4aa00b9f9fe, slot: 708544, graffiti: , service: beacon
Mar 09 21:49:14.800 INFO New block received hash: 0x9b14…f9fe, slot: 708544
Mar 09 21:49:14.984 DEBG Head beacon block slot: 708544, root: 0x9b14…f9fe, finalized_epoch: 22140, finalized_root: 0x28ec…29a7, justified_epoch: 22141, justified_root: 0x59db…e451, service: beacon
Mar 09 21:49:15.055 INFO Unaggregated attestation validator: XXXXX, src: api, slot: 708544, epoch: 22142, delay_ms: 53, index: Y, head: 0xaf5e69de09f384ee3b4fb501458b7000c53bb6758a48817894ec3d2b030e3e6f, service: val_mon
Mar 09 21:49:17.001 DEBG Slot timer sync_state: Synced, current_slot: 708544, head_slot: 708544, head_block: 0x9b14…f9fe, finalized_epoch: 22140, finalized_root: 0x28ec…29a7, peers: 55, service: slot_notifier
```
The reason for this is that the condition was backwards, so that whole block of code was unreachable.
Looking at the attestations for the block included in the block after, we can see that lots of validators missed it. Some of them may be Lighthouse v1.1.1-v1.2.0-rc.0, but it's probable that they would have missed even with the proposer cache primed, given how late the block 708544 arrived (the cache miss occurred 3.787s after the slot start): https://beaconcha.in/block/708545#attestations
## Issue Addressed
NA
## Proposed Changes
Add an optimization to perform `per_slot_processing` from the *leading-edge* of block processing to the *trailing-edge*. Ultimately, this allows us to import the block at slot `n` faster because we used the tail-end of slot `n - 1` to perform `per_slot_processing`.
Additionally, add a "block proposer cache" which allows us to cache the block proposer for some epoch. Since we're now doing trailing-edge `per_slot_processing`, we can prime this cache with the values for the next epoch before those blocks arrive (assuming those blocks don't have some weird forking).
There were several ancillary changes required to achieve this:
- Remove the `state_root` field of `BeaconSnapshot`, since there's no need to know it on a `pre_state` and in all other cases we can just read it from `block.state_root()`.
- This caused some "dust" changes of `snapshot.beacon_state_root` to `snapshot.beacon_state_root()`, where the `BeaconSnapshot::beacon_state_root()` func just reads the state root from the block.
- Rename `types::ShuffingId` to `AttestationShufflingId`. I originally did this because I added a `ProposerShufflingId` struct which turned out to be not so useful. I thought this new name was more descriptive so I kept it.
- Address https://github.com/ethereum/eth2.0-specs/pull/2196
- Add a debug log when we get a block with an unknown parent. There was previously no logging around this case.
- Add a function to `BeaconState` to compute all proposers for an epoch without re-computing the active indices for each slot.
## Additional Info
- ~~Blocked on #2173~~
- ~~Blocked on #2179~~ That PR was wrapped into this PR.
- There's potentially some places where we could avoid computing the proposer indices in `per_block_processing` but I haven't done this here. These would be an optimization beyond the issue at hand (improving block propagation times) and I think this PR is already doing enough. We can come back for that later.
## TODO
- [x] Tidy, improve comments.
- [x] ~~Try avoid computing proposer index in `per_block_processing`?~~
