lib/protocol, lib/discover, lib/db: Use protocol buffer serialization (fixes #3080)

This changes the BEP protocol to use protocol buffer serialization
instead of XDR, and therefore also the database format. The local
discovery protocol is also updated to be protocol buffer format.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3276
LGTM: AudriusButkevicius

lib/db/set.go

@@ -31,9 +31,10 @@ type FileSet struct {
 }
 
 // FileIntf is the set of methods implemented by both protocol.FileInfo and
-// protocol.FileInfoTruncated.
+// FileInfoTruncated.
 type FileIntf interface {
-	Size() int64
+	FileSize() int64
+	FileName() string
 	IsDeleted() bool
 	IsInvalid() bool
 	IsDirectory() bool
@@ -42,7 +43,7 @@ type FileIntf interface {
 }
 
 // The Iterator is called with either a protocol.FileInfo or a
-// protocol.FileInfoTruncated (depending on the method) and returns true to
+// FileInfoTruncated (depending on the method) and returns true to
 // continue iteration, false to stop.
 type Iterator func(f FileIntf) bool
 
@@ -64,7 +65,7 @@ func (s *sizeTracker) addFile(f FileIntf) {
 	} else {
 		s.files++
 	}
-	s.bytes += f.Size()
+	s.bytes += f.FileSize()
 	s.mut.Unlock()
 }
 
@@ -79,7 +80,7 @@ func (s *sizeTracker) removeFile(f FileIntf) {
 	} else {
 		s.files--
 	}
-	s.bytes -= f.Size()
+	s.bytes -= f.FileSize()
 	if s.deleted < 0 || s.files < 0 {
 		panic("bug: removed more than added")
 	}