Memory usage reduction (#1522)

## Issue Addressed

NA

## Proposed Changes

- Adds a new function to allow getting a state with a bad state root history for attestation verification. This reduces unnecessary tree hashing during attestation processing, which accounted for 23% of memory allocations (by bytes) in a recent `heaptrack` observation.
- Don't clone caches on intermediate epoch-boundary states during block processing.
- Reject blocks that are known to fork choice earlier during gossip processing, instead of waiting until after state has been loaded (this only happens in edge-case).
- Avoid multiple re-allocations by creating a "forced" exact size iterator.

## Additional Info

NA

beacon_node/beacon_chain/src/block_verification.rs

@@ -390,9 +390,22 @@ impl<T: BeaconChainTypes> GossipVerifiedBlock<T> {
             });
         }
 
+        let block_root = get_block_root(&block);
+
         // Do not gossip a block from a finalized slot.
         check_block_against_finalized_slot(&block.message, chain)?;
 
+        // Check if the block is already known. We know it is post-finalization, so it is
+        // sufficient to check the fork choice.
+        //
+        // In normal operation this isn't necessary, however it is useful immediately after a
+        // reboot if the `observed_block_producers` cache is empty. In that case, without this
+        // check, we will load the parent and state from disk only to find out later that we
+        // already know this block.
+        if chain.fork_choice.read().contains_block(&block_root) {
+            return Err(BlockError::BlockIsAlreadyKnown);
+        }
+
         // Check that we have not already received a block with a valid signature for this slot.
         if chain
             .observed_block_producers
@@ -415,7 +428,6 @@ impl<T: BeaconChainTypes> GossipVerifiedBlock<T> {
         )?;
 
         let (mut parent, block) = load_parent(block, chain)?;
-        let block_root = get_block_root(&block);
 
         let state = cheap_state_advance_to_obtain_committees(
             &mut parent.beacon_state,
@@ -672,7 +684,10 @@ impl<'a, T: BeaconChainTypes> FullyVerifiedBlock<'a, T> {
                 let state_root = state.update_tree_hash_cache()?;
 
                 let op = if state.slot % T::EthSpec::slots_per_epoch() == 0 {
-                    StoreOp::PutState(state_root.into(), Cow::Owned(state.clone()))
+                    StoreOp::PutState(
+                        state_root.into(),
+                        Cow::Owned(state.clone_with(CloneConfig::committee_caches_only())),
+                    )
                 } else {
                     StoreOp::PutStateSummary(
                         state_root.into(),