lib/protocol, lib/model: Implement high precision time stamps (fixes #3305)

This adds a new nanoseconds field to the FileInfo, populates it during
scans and sets the non-truncated time in Chtimes calls.

The actual file modification time is defined as modified_s seconds +
modified_ns nanoseconds. It's expected that the modified_ns field is <=
1e9 (that is, all whole seconds should go in the modified_s field) but
not really enforced. Given that it's an int32 the timestamp can be
adjusted += ~2.9 seconds by the modified_ns field...

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3431

lib/model/sorter_test.go

@@ -103,7 +103,8 @@ func addFiles(n int, s IndexSorter) {
 			Name:        fmt.Sprintf("file-%d", rnd),
 			Size:        rand.Int63(),
 			Permissions: uint32(rand.Intn(0777)),
-			Modified:    rand.Int63(),
+			ModifiedS:   rand.Int63(),
+			ModifiedNs:  int32(rand.Int63()),
 			Sequence:    rnd,
 			Version:     protocol.Vector{Counters: []protocol.Counter{{ID: 42, Value: uint64(rand.Int63())}}},
 			Blocks: []protocol.BlockInfo{{