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/structs.go

@@ -164,7 +164,7 @@ func (vl VersionList) String() string {
 // update brings the VersionList up to date with file. It returns the updated
 // VersionList, a potentially removed old FileVersion and its index, as well as
 // the index where the new FileVersion was inserted.
-func (vl VersionList) update(folder, device []byte, file protocol.FileInfo, db *instance) (_ VersionList, removedFV FileVersion, removedAt int, insertedAt int) {
+func (vl VersionList) update(folder, device []byte, file protocol.FileInfo, t readOnlyTransaction) (_ VersionList, removedFV FileVersion, removedAt int, insertedAt int) {
 	vl, removedFV, removedAt = vl.pop(device)
 
 	nv := FileVersion{
@@ -198,7 +198,7 @@ func (vl VersionList) update(folder, device []byte, file protocol.FileInfo, db *
 			// to determine the winner.)
 			//
 			// A surprise missing file entry here is counted as a win for us.
-			if of, ok := db.getFile(db.keyer.GenerateDeviceFileKey(nil, folder, vl.Versions[i].Device, []byte(file.Name))); !ok || file.WinsConflict(of) {
+			if of, ok := t.getFile(folder, vl.Versions[i].Device, []byte(file.Name)); !ok || file.WinsConflict(of) {
 				vl = vl.insertAt(i, nv)
 				return vl, removedFV, removedAt, i
 			}