During the refactor of the data layer we had a method that you can use
to do a request/response call i.e. adapter.request > serializer.respond
We weren't sure what to name this but eventually wanted it to live on
the HTTPAdapter itself (see removed comments in the changeset)
We've decided `rpc` is a good name and we've moved this to where we want
it. We've deprecated the old `request` method name but not removed it as
yet. There's also opportunity now to reduce the 'read/write' functions
further as they essentially do the same thing. Again we've left this for
the moment, but we are hoping to get the code for our custom Adapter down
to less than 100 LoC.
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