This gigantic commit ports the existing DB operations in GGTX to use the
transactional API, meaning that we can now compose DB operations and
they will all run in the same Postgres transaction using the same
connection, which will eliminate those class of bugs where concurrent DB
access might result in an inconsistent state.

On top of that, we simplify some parts of the API, for which a summary
is given below:

1. The `NodeStoryEnv` management has been greatly simplified; in the new
   API we don't need an external connection pool to be passed and we
   don't have to pass IO actions, we can just pass DB operations,
   therefore we can greatly simplify the API to just pass mostly pure
   values;

2. Due to the fact that our `DBTx` monad can't do arbitrary IO (which is
   a good thing) we cannot fire Central Exchange notifications
   immediately. Rather that happens now is that we collect the
   `CEMessage` to be sent and we fire them in the relevant concrete
   monad after we finished with the DB transaction. This means that in
   principle there would be a small delay between the DB operation
   taking place and the notification firing but in practice the latency
   should be negligible and bear in mind this is typically what we want:
   if we have a long DB Tx that triggers an error in the middle we don't
   want to be sending out CE messages prematurely if the overall
   operation didn't succeed!

3. There are still a few places in the codebase where we couldn't make
   things fully compositional with regards to the DBTx API, because we
   had Servant handlers which had DB operations mixed with other IO
   effectful computations (or other things like the notification from
   the `MonadJobStatus`). For now we are splitting these functions by
   manually running the partial DB operations, and while this is not
   ideal it can be fixed in subsequent merge requests.

4. The `WorkerEnv` doesn't use `IOException` as its `MonadError`
   anymore, as for consistency we can just use `BackendInternalError` by
   adding a `InternalWorkerError` data constructor accepting the
   `IOException` triggered by the Worker monad.

More testing is needed, with particular attention to performance
(regression) but this should hopefully offer a decent baseline.