eth/downloader: fixes data race between synchronize and other methods (#21201)

* eth/downloaded: fixed datarace between synchronize and Progress

There was a race condition between `downloader.synchronize()` and `Progress` `syncWithPeer` `fetchHeight` `findAncestors` and `processHeaders`
This PR changes the behavior of the downloader a bit.
Previously the functions `Progress` `syncWithPeer` `fetchHeight` `findAncestors` and `processHeaders` read the syncMode anew within their loops. Now they read the syncMode at the start of their function and don't change it during their runtime.

* eth/downloaded: comment

* eth/downloader: added comment

eth/downloader/downloader_test.go

@@ -483,7 +483,7 @@ func assertOwnForkedChain(t *testing.T, tester *downloadTester, common int, leng
 		blocks += length - common
 		receipts += length - common
 	}
-	if tester.downloader.mode == LightSync {
+	if tester.downloader.getMode() == LightSync {
 		blocks, receipts = 1, 1
 	}
 	if hs := len(tester.ownHeaders) + len(tester.ancientHeaders) - 1; hs != headers {

eth/downloader/modes.go

@@ -19,7 +19,8 @@ package downloader
 import "fmt"
 
 // SyncMode represents the synchronisation mode of the downloader.
-type SyncMode int
+// It is a uint32 as it is used with atomic operations.
+type SyncMode uint32
 
 const (
 	FullSync  SyncMode = iota // Synchronise the entire blockchain history from full blocks