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

test/util.go

@@ -455,7 +455,7 @@ func startWalker(dir string, res chan<- fileInfo, abort <-chan struct{}) chan er
 			f = fileInfo{
 				name: rn,
 				mode: info.Mode(),
-				mod:  info.ModTime().Unix(),
+				mod:  info.ModTime().Truncate(time.Microsecond).UnixNano(),
 				size: info.Size(),
 			}
 			sum, err := md5file(path)