Prior to this, the search is still racy but one tends to notice
this only when the DB is large or network is involved.
The user can initiate a search, get bored, navigate to another chan
issue a different search.
Now however, the results of the first search come back in and
hilarity ensues as we are now confused with the state.
To avoid this, keep track of the last search done and any result
that comes in that isn't equal to the active query is garbage and
can be dropped.
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
