core,eth: add debug_setTrieFlushInterval to change trie flush frequency (#24785)

This PR makes it possible to modify the flush interval time via RPC. On one extreme, `0s`, it would act as an archive node. If set to `1h`, means that after one hour of effective block processing time, the trie would be flushed. If one block takes 200ms, this means that a flush would occur every `5*3600=18000`  blocks -- however, if the memory size of the cached states grows too large, it will flush sooner. 

Essentially, this makes it possible to configure the node to be more or less "archive:ish", and without restarting the node while reconfiguring it.

eth/api.go

@@ -590,3 +590,14 @@ func (api *DebugAPI) GetAccessibleState(from, to rpc.BlockNumber) (uint64, error
 	}
 	return 0, errors.New("no state found")
 }
+
+// SetTrieFlushInterval configures how often in-memory tries are persisted
+// to disk. The value is in terms of block processing time, not wall clock.
+func (api *DebugAPI) SetTrieFlushInterval(interval string) error {
+	t, err := time.ParseDuration(interval)
+	if err != nil {
+		return err
+	}
+	api.eth.blockchain.SetTrieFlushInterval(t)
+	return nil
+}