lib/db: Fix iterating sequence index (fixes #5340) (#5462)

There was a problem in iterating the sequence index that could result
in missing updates. The issue is that while the index was (correctly)
iterated in a snapshot, the actual file infos were read dirty outside of
the snapshot. This fixes this by doing the reads inside the snapshot,
and also updates a couple of other places that did the same thing more
or less harmfully (I didn't investigate).

To avoid similar issues in the future I did some renaming of the
getFile* methods - the ones in a transaction are just getFile, while the
ones directly on the database are variants of getFileDirty to highlight
what's going on.

lib/db/db_test.go

@@ -149,7 +149,7 @@ func TestUpdate0to3(t *testing.T) {
 
 	updater.updateSchema0to1()
 
-	if _, ok := db.getFile(db.keyer.GenerateDeviceFileKey(nil, folder, protocol.LocalDeviceID[:], []byte(slashPrefixed))); ok {
+	if _, ok := db.getFileDirty(folder, protocol.LocalDeviceID[:], []byte(slashPrefixed)); ok {
 		t.Error("File prefixed by '/' was not removed during transition to schema 1")
 	}