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/protocol/errors.go

@@ -6,13 +6,6 @@ import (
 	"errors"
 )
 
-const (
-	ecNoError int32 = iota
-	ecGeneric
-	ecNoSuchFile
-	ecInvalid
-)
-
 var (
 	ErrNoError    error
 	ErrGeneric    = errors.New("generic error")
@@ -20,32 +13,32 @@ var (
 	ErrInvalid    = errors.New("file is invalid")
 )
 
-var lookupError = map[int32]error{
-	ecNoError:    ErrNoError,
-	ecGeneric:    ErrGeneric,
-	ecNoSuchFile: ErrNoSuchFile,
-	ecInvalid:    ErrInvalid,
+var lookupError = map[ErrorCode]error{
+	ErrorCodeNoError:     ErrNoError,
+	ErrorCodeGeneric:     ErrGeneric,
+	ErrorCodeNoSuchFile:  ErrNoSuchFile,
+	ErrorCodeInvalidFile: ErrInvalid,
 }
 
-var lookupCode = map[error]int32{
-	ErrNoError:    ecNoError,
-	ErrGeneric:    ecGeneric,
-	ErrNoSuchFile: ecNoSuchFile,
-	ErrInvalid:    ecInvalid,
+var lookupCode = map[error]ErrorCode{
+	ErrNoError:    ErrorCodeNoError,
+	ErrGeneric:    ErrorCodeGeneric,
+	ErrNoSuchFile: ErrorCodeNoSuchFile,
+	ErrInvalid:    ErrorCodeInvalidFile,
 }
 
-func codeToError(errcode int32) error {
-	err, ok := lookupError[errcode]
+func codeToError(code ErrorCode) error {
+	err, ok := lookupError[code]
 	if !ok {
 		return ErrGeneric
 	}
 	return err
 }
 
-func errorToCode(err error) int32 {
+func errorToCode(err error) ErrorCode {
 	code, ok := lookupCode[err]
 	if !ok {
-		return ecGeneric
+		return ErrorCodeGeneric
 	}
 	return code
 }