Turn the relevant chain into a Vec before using it

Some checks use the same blocks, so we take a copy of the block borrows
before using them. That way, we don't have to manage the position of the
iterator between checks.

zebra-state/src/service/check.rs

@@ -47,10 +47,15 @@ where
         .expect("finalized state must contain at least one block to do contextual validation");
     check::block_is_not_orphaned(finalized_tip_height, prepared.height)?;
 
-    // Peek at the first block
-    let mut relevant_chain = relevant_chain.into_iter().peekable();
+    // The maximum number of blocks used by contextual checks
+    const MAX_CONTEXT_BLOCKS: usize = POW_AVERAGING_WINDOW + POW_MEDIAN_BLOCK_SPAN;
+    let relevant_chain: Vec<_> = relevant_chain
+        .into_iter()
+        .take(MAX_CONTEXT_BLOCKS)
+        .collect();
+
     let parent_block = relevant_chain
-        .peek()
+        .get(0)
         .expect("state must contain parent block to do contextual validation");
     let parent_block = parent_block.borrow();
     let parent_height = parent_block
@@ -58,18 +63,13 @@ where
         .expect("valid blocks have a coinbase height");
     check::height_one_more_than_parent_height(parent_height, prepared.height)?;
 
-    // Note: the difficulty check reads the first 28 blocks from the relevant
-    // chain iterator. If you want to use those blocks in other checks, you'll
-    // need to clone them here.
-
     if relevant_chain.len() >= POW_AVERAGING_WINDOW + POW_MEDIAN_BLOCK_SPAN {
-        let relevant_data = relevant_chain.map(|block| {
+        let relevant_data = relevant_chain.iter().map(|block| {
             (
                 block.borrow().header.difficulty_threshold,
                 block.borrow().header.time,
             )
         });
-        // Reads the first 28 blocks from the iterator
         let expected_difficulty =
             AdjustedDifficulty::new_from_block(&prepared.block, network, relevant_data);
         check::difficulty_threshold_is_valid(