The only reason we accepted a client was that so we have access
to the next message id when we need it.
So let's accept an id provider function instead.
Message stores are more complicated that a sync "fire and forget"
API allows for.
For starters, non trivial stores (say sqlite) can fail during init
and we want to be able to catch that.
Second, we really need to be able to run migrations and such, which
may block (and fail) the activation of the store.
On the plus side, this pushes error handling to the caller rather
than the stores, which is a good thing as that allows us to eventually
push this to the client in the UI, rather than just logging it in the
server on stdout
