rpc: Avoid using Pool since it conflicts with http2 (#7467)

A race is detected between a bytes.Buffer generated with cmd/rpc.Pool
and http2 module. An issue is raised in golang (https://github.com/golang/go/issues/31192).

Meanwhile, this commit disables Pool in RPC code and it generates a
new 1kb of bytes.Buffer for each RPC call.

cmd/rpc/server_test.go

@@ -253,8 +253,7 @@ func TestServerCall(t *testing.T) {
 	}
 
 	for i, testCase := range testCases {
-		buf := bufPool.Get()
-		defer bufPool.Put(buf)
+		buf := bytes.NewBuffer([]byte{})
 
 		err := testCase.server.call(testCase.serviceMethod, testCase.argBytes, buf)
 		expectErr := (err != nil)