It is reported in https://github.com/grpc/grpc/issues/32356 that there
is a race on vptr for `UnimplementedAsyncRequest` which would cause
crashes for multi-threaded server if clients send unimplemented RPC
request to the server.
The cause is that the server requests a call for
`UnimplementedAsyncRequest` in its base class `GenericAsyncRequest` when
the `vptr` still points to the base class's `vtable`. If the call went
in and another server thread picks up the tag before the `vptr` points
back to the derived class's `vtable`, it would call the wrong virtual
function and also this is a data race. This fix makes the request of the
call inside the derived class's constructor.
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