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/structs.proto

@@ -0,0 +1,35 @@
+syntax = "proto3";
+
+package db;
+
+import "github.com/gogo/protobuf/gogoproto/gogo.proto";
+import "github.com/syncthing/syncthing/lib/protocol/bep.proto";
+
+option (gogoproto.goproto_getters_all) = false;
+option (gogoproto.sizer_all) = false;
+option (gogoproto.protosizer_all) = true;
+
+message FileVersion {
+    protocol.Vector version = 1 [(gogoproto.nullable) = false];
+    bytes           device  = 2;
+}
+
+message VersionList {
+    option (gogoproto.goproto_stringer) = false;
+    repeated FileVersion versions = 1 [(gogoproto.nullable) = false];
+}
+
+// Must be the same as FileInfo but without the blocks field
+message FileInfoTruncated {
+    option (gogoproto.goproto_stringer) = false;
+    string                name           = 1;
+    protocol.FileInfoType type           = 2;
+    int64                 size           = 3;
+    uint32                permissions    = 4;
+    int64                 modified       = 5;
+    bool                  deleted        = 6;
+    bool                  invalid        = 7;
+    bool                  no_permissions = 8;
+    protocol.Vector       version        = 9 [(gogoproto.nullable) = false];
+    int64                 local_version  = 10;
+}