eth: request id dispatcher and direct req/reply APIs (#23576)

* eth: request ID based message dispatcher

* eth: fix dispatcher cancellation, rework fetchers idleness tracker

* eth/downloader: drop peers who refuse to serve advertised chains

eth/downloader/modes.go

@@ -24,7 +24,6 @@ type SyncMode uint32
 
 const (
 	FullSync  SyncMode = iota // Synchronise the entire blockchain history from full blocks
-	FastSync                  // Quickly download the headers, full sync only at the chain
 	SnapSync                  // Download the chain and the state via compact snapshots
 	LightSync                 // Download only the headers and terminate afterwards
 )
@@ -38,8 +37,6 @@ func (mode SyncMode) String() string {
 	switch mode {
 	case FullSync:
 		return "full"
-	case FastSync:
-		return "fast"
 	case SnapSync:
 		return "snap"
 	case LightSync:
@@ -53,8 +50,6 @@ func (mode SyncMode) MarshalText() ([]byte, error) {
 	switch mode {
 	case FullSync:
 		return []byte("full"), nil
-	case FastSync:
-		return []byte("fast"), nil
 	case SnapSync:
 		return []byte("snap"), nil
 	case LightSync:
@@ -68,14 +63,12 @@ func (mode *SyncMode) UnmarshalText(text []byte) error {
 	switch string(text) {
 	case "full":
 		*mode = FullSync
-	case "fast":
-		*mode = FastSync
 	case "snap":
 		*mode = SnapSync
 	case "light":
 		*mode = LightSync
 	default:
-		return fmt.Errorf(`unknown sync mode %q, want "full", "fast" or "light"`, text)
+		return fmt.Errorf(`unknown sync mode %q, want "full", "snap" or "light"`, text)
 	}
 	return nil
 }