Enable proposer boost re-orging (#2860)

## Proposed Changes

With proposer boosting implemented (#2822) we have an opportunity to re-org out late blocks.

This PR adds three flags to the BN to control this behaviour:

* `--disable-proposer-reorgs`: turn aggressive re-orging off (it's on by default).
* `--proposer-reorg-threshold N`: attempt to orphan blocks with less than N% of the committee vote. If this parameter isn't set then N defaults to 20% when the feature is enabled.
* `--proposer-reorg-epochs-since-finalization N`: only attempt to re-org late blocks when the number of epochs since finalization is less than or equal to N. The default is 2 epochs, meaning re-orgs will only be attempted when the chain is finalizing optimally.

For safety Lighthouse will only attempt a re-org under very specific conditions:

1. The block being proposed is 1 slot after the canonical head, and the canonical head is 1 slot after its parent. i.e. at slot `n + 1` rather than building on the block from slot `n` we build on the block from slot `n - 1`.
2. The current canonical head received less than N% of the committee vote. N should be set depending on the proposer boost fraction itself, the fraction of the network that is believed to be applying it, and the size of the largest entity that could be hoarding votes.
3. The current canonical head arrived after the attestation deadline from our perspective. This condition was only added to support suppression of forkchoiceUpdated messages, but makes intuitive sense.
4. The block is being proposed in the first 2 seconds of the slot. This gives it time to propagate and receive the proposer boost.


## Additional Info

For the initial idea and background, see: https://github.com/ethereum/consensus-specs/pull/2353#issuecomment-950238004

There is also a specification for this feature here: https://github.com/ethereum/consensus-specs/pull/3034

Co-authored-by: Michael Sproul <micsproul@gmail.com>
Co-authored-by: pawan <pawandhananjay@gmail.com>

beacon_node/beacon_chain/src/chain_config.rs

@@ -1,9 +1,18 @@
-pub use proto_array::CountUnrealizedFull;
+pub use proto_array::{CountUnrealizedFull, ReOrgThreshold};
 use serde_derive::{Deserialize, Serialize};
-use types::Checkpoint;
+use std::time::Duration;
+use types::{Checkpoint, Epoch};
 
+pub const DEFAULT_RE_ORG_THRESHOLD: ReOrgThreshold = ReOrgThreshold(20);
+pub const DEFAULT_RE_ORG_MAX_EPOCHS_SINCE_FINALIZATION: Epoch = Epoch::new(2);
 pub const DEFAULT_FORK_CHOICE_BEFORE_PROPOSAL_TIMEOUT: u64 = 250;
 
+/// Default fraction of a slot lookahead for payload preparation (12/3 = 4 seconds on mainnet).
+pub const DEFAULT_PREPARE_PAYLOAD_LOOKAHEAD_FACTOR: u32 = 3;
+
+/// Fraction of a slot lookahead for fork choice in the state advance timer (500ms on mainnet).
+pub const FORK_CHOICE_LOOKAHEAD_FACTOR: u32 = 24;
+
 #[derive(Debug, PartialEq, Eq, Clone, Deserialize, Serialize)]
 pub struct ChainConfig {
     /// Maximum number of slots to skip when importing a consensus message (e.g., block,
@@ -21,6 +30,10 @@ pub struct ChainConfig {
     pub enable_lock_timeouts: bool,
     /// The max size of a message that can be sent over the network.
     pub max_network_size: usize,
+    /// Maximum percentage of committee weight at which to attempt re-orging the canonical head.
+    pub re_org_threshold: Option<ReOrgThreshold>,
+    /// Maximum number of epochs since finalization for attempting a proposer re-org.
+    pub re_org_max_epochs_since_finalization: Epoch,
     /// Number of milliseconds to wait for fork choice before proposing a block.
     ///
     /// If set to 0 then block proposal will not wait for fork choice at all.
@@ -47,6 +60,11 @@ pub struct ChainConfig {
     pub count_unrealized_full: CountUnrealizedFull,
     /// Optionally set timeout for calls to checkpoint sync endpoint.
     pub checkpoint_sync_url_timeout: u64,
+    /// The offset before the start of a proposal slot at which payload attributes should be sent.
+    ///
+    /// Low values are useful for execution engines which don't improve their payload after the
+    /// first call, and high values are useful for ensuring the EL is given ample notice.
+    pub prepare_payload_lookahead: Duration,
 }
 
 impl Default for ChainConfig {
@@ -57,6 +75,8 @@ impl Default for ChainConfig {
             reconstruct_historic_states: false,
             enable_lock_timeouts: true,
             max_network_size: 10 * 1_048_576, // 10M
+            re_org_threshold: Some(DEFAULT_RE_ORG_THRESHOLD),
+            re_org_max_epochs_since_finalization: DEFAULT_RE_ORG_MAX_EPOCHS_SINCE_FINALIZATION,
             fork_choice_before_proposal_timeout_ms: DEFAULT_FORK_CHOICE_BEFORE_PROPOSAL_TIMEOUT,
             // Builder fallback configs that are set in `clap` will override these.
             builder_fallback_skips: 3,
@@ -68,6 +88,7 @@ impl Default for ChainConfig {
             paranoid_block_proposal: false,
             count_unrealized_full: CountUnrealizedFull::default(),
             checkpoint_sync_url_timeout: 60,
+            prepare_payload_lookahead: Duration::from_secs(4),
         }
     }
 }