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/fs/mtimefs.go

@@ -4,9 +4,6 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this file,
 // You can obtain one at http://mozilla.org/MPL/2.0/.
 
-//go:generate go run ../../script/protofmt.go mtime.proto
-//go:generate protoc --proto_name=../../../../../:../../../../gogo/protobuf/protobuf:. --gogofast_out=. mtime.proto
-
 package fs
 
 import (