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/schemaupdater.go

@@ -203,7 +203,7 @@ func (db *schemaUpdater) updateSchema2to3() {
 			name := []byte(f.FileName())
 			dk = db.keyer.GenerateDeviceFileKey(dk, folder, protocol.LocalDeviceID[:], name)
 			var v protocol.Vector
-			haveFile, ok := db.getFileTrunc(dk, true)
+			haveFile, ok := t.getFileTrunc(dk, true)
 			if ok {
 				v = haveFile.FileVersion()
 			}