Builder profit threshold flag (#3534)

## Issue Addressed

Resolves https://github.com/sigp/lighthouse/issues/3517

## Proposed Changes

Adds a `--builder-profit-threshold <wei value>` flag to the BN. If an external payload's value field is less than this value, the local payload will be used. The value of the local payload will not be checked (it can't really be checked until the engine API is updated to support this).


Co-authored-by: realbigsean <sean@sigmaprime.io>

beacon_node/execution_layer/src/test_utils/mock_builder.rs

@@ -33,7 +33,7 @@ use types::{
 pub enum Operation {
     FeeRecipient(Address),
     GasLimit(usize),
-    Value(usize),
+    Value(Uint256),
     ParentHash(Hash256),
     PrevRandao(Hash256),
     BlockNumber(usize),
@@ -47,7 +47,7 @@ impl Operation {
                 bid.header.fee_recipient = to_ssz_rs(&fee_recipient)?
             }
             Operation::GasLimit(gas_limit) => bid.header.gas_limit = gas_limit as u64,
-            Operation::Value(value) => bid.value = to_ssz_rs(&Uint256::from(value))?,
+            Operation::Value(value) => bid.value = to_ssz_rs(&value)?,
             Operation::ParentHash(parent_hash) => bid.header.parent_hash = to_ssz_rs(&parent_hash)?,
             Operation::PrevRandao(prev_randao) => bid.header.prev_randao = to_ssz_rs(&prev_randao)?,
             Operation::BlockNumber(block_number) => bid.header.block_number = block_number as u64,
@@ -149,7 +149,9 @@ impl<E: EthSpec> MockBuilder<E> {
     }
 
     pub fn add_operation(&self, op: Operation) {
-        self.operations.write().push(op);
+        // Insert operations at the front of the vec to make sure `apply_operations` applies them
+        // in the order they are added.
+        self.operations.write().insert(0, op);
     }
 
     pub fn invalid_signatures(&self) {