ttrpc provides WithOnClose option for user and ttrpc will call the
callback function when connection is closed unexpectedly or the ttrpc
client's Close() method is called. containerd runtime plugin uses it
to handle cleanup the resources created by containerd shim.
But the ttrpc client's Close() is only trigger and the shim's cleanup
resource callback is called asynchronously, which might make part of
resources leaky. There is an example from containerd-runtime-v2 for
runc:
```happy
[Task.Delete goroutine] [cleanupCallback goroutine]
call ttrpc client.Close() -->
read bundle and call runc delete
delete bundle
```
If the cleanupCallback is called after deleting bundle, the callback
will fail to call runc delete. If there is any running processes, the
resource becomes leaky.
```unhappy
[Task.Delete goroutine] [cleanupCallback goroutine]
call ttrpc client.Close() -->
delete bundle
failed to read bundle and call runc delete
```
In order to avoid this, introduces the UserOnCloseWait to make sure that
the cleanupCallback has been called synchronously, like:
```
[Task.Delete goroutine] [cleanupCallback goroutine]
call ttrpc client.Close() -->
wait for callback
read bundle and call runc delete
<-- finish sync
delete bundle
```
Signed-off-by: Wei Fu <fuweid89@gmail.com>
