This enables db migrations to be undone, or "down migrated".
The down migration shouldn't be done automatically
as it could lead to severe data loss if that were done.
Hence, we still hard fail if we encounter a version lower than what
we have in the DB.
A CLI will be added in a later commit that allows users to explicitly
do that.
This sets up the testing infrastructure to test migrations we are
doing.
It's done on a in memory database directly, we are only interested
in the statements themselves and it's easier than to try and
inject a prepared db into the store.
We do add some dummy data though to make sure we actually execute
the things as we expect.
This just repeats the hard coded values from the code, which
is not helping.
We need to touch that test whenever we modify the sql which is
undesired and it doesn't test any useful functionality.
Any error that may ensue would hopefully be tracked by the other
test.
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.
TL is stupid and doesn't wait for message{Provider,Storage} to
settle before it starts using the store.
While this should be fixed globally, we can hack around the problem
by pushing everything onto the call stack and hope that we'll eventually
finish the setup before we blow the stack.
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
