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/WhatcdResolver.hs

@@ -688,7 +688,7 @@ runAppWith appT = withTracer $ \tracer -> withDb $ \db -> do
         {- resource destruction -} Postgres.close
         {- unusedResourceOpenTime -} 10
         {- max resources across all stripes -} 20
-  transmissionSessionId <- newEmptyMVar
+  transmissionSessionId <- newIORef Nothing
   let newAppT = do
         logInfo [fmt|Running with config: {showPretty config}|]
         logInfo [fmt|Connected to database at {db & TmpPg.toDataDirectory} on socket {db & TmpPg.toConnectionString}|]