Merge PR #2800: Limit total number of signatures per transaction

* Limit total number of signatures per transaction
* Fail if limit is exceeded
* Loop over all sigs and count subkeys
* No need for a type switch, adopt early return
* Test rejection logic
* Mention sigs limit

types/errors.go

@@ -57,6 +57,7 @@ const (
 	CodeOutOfGas          CodeType = 12
 	CodeMemoTooLarge      CodeType = 13
 	CodeInsufficientFee   CodeType = 14
+	CodeTooManySignatures CodeType = 15
 
 	// CodespaceRoot is a codespace for error codes in this file only.
 	// Notice that 0 is an "unset" codespace, which can be overridden with
@@ -103,6 +104,8 @@ func CodeToDefaultMsg(code CodeType) string {
 		return "memo too large"
 	case CodeInsufficientFee:
 		return "insufficient fee"
+	case CodeTooManySignatures:
+		return "maximum numer of signatures exceeded"
 	default:
 		return unknownCodeMsg(code)
 	}
@@ -155,6 +158,9 @@ func ErrMemoTooLarge(msg string) Error {
 func ErrInsufficientFee(msg string) Error {
 	return newErrorWithRootCodespace(CodeInsufficientFee, msg)
 }
+func ErrTooManySignatures(msg string) Error {
+	return newErrorWithRootCodespace(CodeTooManySignatures, msg)
+}
 
 //----------------------------------------
 // Error & sdkError