This patch drops several uses of the RpcCallback compatibility class.
All of these were trivial and straight forward conversions so I
batched them up.
There are still several other uses of RpcCallback, but the conversions
are were not necessarily trivial, and may be broken in one way or
another, so I wanted to address them separately. In particular, the
use of mixin classes means that there could be cases where the version
declaration is being stepped on, so they need to be investigated more
closely.
Part of blueprint drop-rpc-compat.
Change-Id: I0977aee863d4fcc6a14a025215c6e41fe38a87f0
9897b1aca8