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/model/rwfolder_test.go

@@ -50,10 +50,8 @@ func setUpFile(filename string, blockNumbers []int) protocol.FileInfo {
 	}
 
 	return protocol.FileInfo{
-		Name:     filename,
-		Flags:    0,
-		Modified: 0,
-		Blocks:   existingBlocks,
+		Name:   filename,
+		Blocks: existingBlocks,
 	}
 }