feat(en): Cache blocks in fetch_l2_block (#403)

# What ❔

- `fetch_l2_block` saves blocks in cache
- fixed log in `ExternalIO:: seal_miniblock`

## Why ❔

Making cache client to behave traditional way, i.e. cache results got
from server. This will also fix invariant that previous block is always
(except for first iteration after start) kept in cache and fix sporadic
panics caused by inconsistent API responses
 
## Checklist

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- [x] PR title corresponds to the body of PR (we generate changelog
entries from PRs).
- [ ] Tests for the changes have been added / updated.
- [x] Documentation comments have been added / updated.
- [x] Code has been formatted via `zk fmt` and `zk lint`.

core/lib/zksync_core/src/sync_layer/client.rs

@@ -182,18 +182,19 @@ impl CachingMainNodeClient {
 
     /// Cached version of [`HttpClient::sync_l2_block`].
     pub async fn fetch_l2_block(
-        &self,
+        &mut self,
         miniblock: MiniblockNumber,
     ) -> anyhow::Result<Option<SyncBlock>> {
-        let block = self.blocks.get(&miniblock).cloned();
         FETCHER_METRICS.cache_total[&CachedMethod::SyncL2Block].inc();
-        match block {
-            Some(block) => {
-                FETCHER_METRICS.cache_hit[&CachedMethod::SyncL2Block].inc();
-                Ok(Some(block))
-            }
-            None => self.client.fetch_l2_block(miniblock, true).await,
+        if let Some(block) = self.blocks.get(&miniblock).cloned() {
+            FETCHER_METRICS.cache_hit[&CachedMethod::SyncL2Block].inc();
+            return Ok(Some(block));
+        }
+        let block = self.client.fetch_l2_block(miniblock, true).await?;
+        if let Some(block) = block.clone() {
+            self.blocks.insert(miniblock, block);
         }
+        Ok(block)
     }
 
     /// Re-export of [`MainNodeClient::fetch_l2_block_number()`]. Added to not expose the internal client.

core/lib/zksync_core/src/sync_layer/external_io.rs

@@ -543,8 +543,8 @@ impl StateKeeperIO for ExternalIO {
 
         self.sync_state
             .set_local_block(self.current_miniblock_number);
-        self.current_miniblock_number += 1;
         tracing::info!("Miniblock {} is sealed", self.current_miniblock_number);
+        self.current_miniblock_number += 1;
     }
 
     async fn seal_l1_batch(