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

vendor/github.com/gogo/protobuf/test/importdedup/proto.pb.go

@@ -0,0 +1,69 @@
+// Code generated by protoc-gen-gogo.
+// source: proto.proto
+// DO NOT EDIT!
+
+/*
+Package importdedup is a generated protocol buffer package.
+
+It is generated from these files:
+	proto.proto
+
+It has these top-level messages:
+	Object
+*/
+package importdedup
+
+import proto "github.com/gogo/protobuf/proto"
+import fmt "fmt"
+import math "math"
+import _ "github.com/gogo/protobuf/gogoproto"
+import subpkg "github.com/gogo/protobuf/test/importdedup/subpkg"
+
+import github_com_gogo_protobuf_test_importdedup_subpkg "github.com/gogo/protobuf/test/importdedup/subpkg"
+
+// Reference imports to suppress errors if they are not otherwise used.
+var _ = proto.Marshal
+var _ = fmt.Errorf
+var _ = math.Inf
+
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the proto package it is being compiled against.
+const _ = proto.GoGoProtoPackageIsVersion1
+
+type Object struct {
+	CustomField      *github_com_gogo_protobuf_test_importdedup_subpkg.CustomType `protobuf:"bytes,1,opt,name=CustomField,json=customField,customtype=github.com/gogo/protobuf/test/importdedup/subpkg.CustomType" json:"CustomField,omitempty"`
+	SubObject        *subpkg.SubObject                                            `protobuf:"bytes,2,opt,name=SubObject,json=subObject" json:"SubObject,omitempty"`
+	XXX_unrecognized []byte                                                       `json:"-"`
+}
+
+func (m *Object) Reset()                    { *m = Object{} }
+func (m *Object) String() string            { return proto.CompactTextString(m) }
+func (*Object) ProtoMessage()               {}
+func (*Object) Descriptor() ([]byte, []int) { return fileDescriptorProto, []int{0} }
+
+func (m *Object) GetSubObject() *subpkg.SubObject {
+	if m != nil {
+		return m.SubObject
+	}
+	return nil
+}
+
+func init() {
+	proto.RegisterType((*Object)(nil), "importdedup.Object")
+}
+
+var fileDescriptorProto = []byte{
+	// 178 bytes of a gzipped FileDescriptorProto
+	0x1f, 0x8b, 0x08, 0x00, 0x00, 0x09, 0x6e, 0x88, 0x02, 0xff, 0xe2, 0xe2, 0x2e, 0x28, 0xca, 0x2f,
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+	0x00, 0x00,
+}