In #25499, the user code in rpc/remote will be execute right away when the callee receives the call. But as agreed in the protocol #26759, the user code should not run until all RRefs are confirmed by the owner, to make sure that reference counting is correctly handled even if user code triggers failures.

cc @ezyang @gchanan @zou3519 @jerryzh168 @pietern @mrshenli @pritamdamania87 @zhaojuanmao @satgera @rohan-varma @gqchen @aazzolini @xush6528