core, trie: intermediate mempool between trie and database (#15857)

This commit reduces database I/O by not writing every state trie to disk.

core/chain_indexer.go

@@ -203,6 +203,9 @@ func (c *ChainIndexer) eventLoop(currentHeader *types.Header, events chan ChainE
 			if header.ParentHash != prevHash {
 				// Reorg to the common ancestor (might not exist in light sync mode, skip reorg then)
 				// TODO(karalabe, zsfelfoldi): This seems a bit brittle, can we detect this case explicitly?
+
+				// TODO(karalabe): This operation is expensive and might block, causing the event system to
+				// potentially also lock up. We need to do with on a different thread somehow.
 				if h := FindCommonAncestor(c.chainDb, prevHeader, header); h != nil {
 					c.newHead(h.Number.Uint64(), true)
 				}