RPC: eliminate NO_RESPONSE

Using a sentinel value in the response-id is ambiguous because the
msgpack-rpc spec allows all values (including zero/max). And clients
control the id, so we can't be sure they won't use the sentinel value.

Instead of a sentinel value, check the message type explicitly.

ref #8850

src/nvim/api/private/defs.h

@@ -29,14 +29,13 @@ typedef enum {
 } ErrorType;
 
 typedef enum {
-  kMessageTypeRequest,
-  kMessageTypeResponse,
-  kMessageTypeNotification
+  kMessageTypeUnknown = -1,
+  // Per msgpack-rpc spec.
+  kMessageTypeRequest = 0,
+  kMessageTypeResponse = 1,
+  kMessageTypeNotification = 2,
 } MessageType;
 
-/// Used as the message ID of notifications.
-#define NO_RESPONSE UINT64_MAX
-
 /// Mask for all internal calls
 #define INTERNAL_CALL_MASK (((uint64_t)1) << (sizeof(uint64_t) * 8 - 1))