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/protocol/protocol_test.go

@@ -221,7 +221,7 @@ func TestMarshalledIndexMessageSize(t *testing.T) {
 		Type:        FileInfoTypeFile,
 		Size:        fileSize,
 		Permissions: 0666,
-		Modified:    time.Now().Unix(),
+		ModifiedS:   time.Now().Unix(),
 		Version:     Vector{Counters: []Counter{{ID: 1 << 60, Value: 1}, {ID: 2 << 60, Value: 1}}},
 		Blocks:      make([]BlockInfo, fileSize/blockSize),
 	}