fix(users/Profpatsch/whatcd-resolver): fix transmission session

The logic around transmission session handling was f*cked, this fixes
that.

We use an IORef instead of an MVar, since we want to unconditionally
write the new value. Even if multiple requests race, I *hope* that
transmission returns the same session id, otherwise we might get a
request loop. But it should be fine. (The semantics is not nicely
documented in the RPC docs.)

Additionally, log the session ids in the requests.

Change-Id: Id7d33f8cb74cb349e502331cad5eb5abe8a624cd
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11673
Autosubmit: Profpatsch <mail@profpatsch.de>
Reviewed-by: Profpatsch <mail@profpatsch.de>
Tested-by: BuildkiteCI

users/Profpatsch/whatcd-resolver/src/AppT.hs

@@ -27,7 +27,7 @@ data Context = Context
     tracer :: Otel.Tracer,
     pgFormat :: PgFormatPool,
     pgConnPool :: Pool Postgres.Connection,
-    transmissionSessionId :: MVar ByteString
+    transmissionSessionId :: IORef (Maybe ByteString)
   }
 
 newtype AppT m a = AppT {unAppT :: ReaderT Context m a}