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/enumprefix/enumprefix.pb.go

@@ -0,0 +1,64 @@
+// Code generated by protoc-gen-gogo.
+// source: enumprefix.proto
+// DO NOT EDIT!
+
+/*
+	Package enumprefix is a generated protocol buffer package.
+
+	It is generated from these files:
+		enumprefix.proto
+
+	It has these top-level messages:
+		MyMessage
+*/
+package enumprefix
+
+import proto "github.com/gogo/protobuf/proto"
+import fmt "fmt"
+import math "math"
+import test "github.com/gogo/protobuf/test"
+import _ "github.com/gogo/protobuf/gogoproto"
+
+// 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 MyMessage struct {
+	TheField         test.TheTestEnum `protobuf:"varint,1,opt,name=TheField,json=theField,enum=test.TheTestEnum" json:"TheField"`
+	XXX_unrecognized []byte           `json:"-"`
+}
+
+func (m *MyMessage) Reset()                    { *m = MyMessage{} }
+func (m *MyMessage) String() string            { return proto.CompactTextString(m) }
+func (*MyMessage) ProtoMessage()               {}
+func (*MyMessage) Descriptor() ([]byte, []int) { return fileDescriptorEnumprefix, []int{0} }
+
+func (m *MyMessage) GetTheField() test.TheTestEnum {
+	if m != nil {
+		return m.TheField
+	}
+	return test.A
+}
+
+func init() {
+	proto.RegisterType((*MyMessage)(nil), "enumprefix.MyMessage")
+}
+
+var fileDescriptorEnumprefix = []byte{
+	// 148 bytes of a gzipped FileDescriptorProto
+	0x1f, 0x8b, 0x08, 0x00, 0x00, 0x09, 0x6e, 0x88, 0x02, 0xff, 0xe2, 0x12, 0x48, 0xcd, 0x2b, 0xcd,
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+}