Support { w : 0 } when using write commands

We used to support { w : 0 } by using the older insert/update/delete opcodes.

Now that we have removed support for those older opcodes we need to support { w : 0 } when using write commands.

We need to consume and discard the skeleton response from the server when using { w : 0 } but the application using { w : 0 } should not block waiting for the response.