perf(storage): batch trie updates across blocks in save_blocks

[gakonst]

Summary

Batches trie updates across all blocks in save_blocks instead of writing per-block.

Problem

Per #eng-perf profiling, write_trie_updates was taking ~25% of persistence time. The current implementation calls write_trie_updates_sorted once per block, opening/closing cursors N times.

In back-to-back (b2b) scenarios with 75-250 accumulated blocks, this overhead compounds significantly.

Solution

Accumulate trie updates across blocks using the existing extend_ref method, then write them all in a single batch:

// Accumulate across blocks
let mut accumulated_trie_updates: Option<TrieUpdatesSorted> = None;

for block in blocks {
    // ... other per-block writes ...
    
    match &mut accumulated_trie_updates {
        Some(acc) => acc.extend_ref(&trie_data.trie_updates),
        None => accumulated_trie_updates = Some((*trie_data.trie_updates).clone()),
    }
}

// Single batch write at end
if let Some(trie_updates) = &accumulated_trie_updates {
    self.write_trie_updates_sorted(trie_updates)?;
}

Expected Impact

• ~50% reduction in write_trie_updates time for b2b scenarios
• Reduces cursor open/close overhead from N to 1
• Reduces MDBX transaction overhead

Testing

• All existing reth-provider tests pass

Related

• Based on analysis from #eng-perf Slack channel
• Complements georgios/history-indices-from-memory (26% history indices optimization)
• Supersedes perf(provider): flatten trie updates before persisting #21106 and perf(storage): batch trie updates across blocks in save_blocks #21139

[mediocregopher]

Closes #20611

[mattsse]

this now only has benches left