ChainDB: minor tidying in Reader.forward

This diff combines two cheap STM transactions instead of running one
immediately after the other. I'm not sure the difference matters, but this
seems more likely to be correct.

ouroboros-consensus/src/Ouroboros/Consensus/Storage/ChainDB/Impl/Reader.hs

@@ -415,14 +415,15 @@ forward ::
   -> [Point blk]
   -> m (Maybe (Point blk))
 forward registry varReader blockComponent CDB{..} = \pts -> do
-    -- NOTE: we don't use 'Query.getCurrentChain', which only returns the last
-    -- @k@ headers, because we want to see the headers that have not yet been
-    -- written to the ImmutableDB too.
-    (curChain, readerState) <- atomically $
-      (,) <$> readTVar cdbChain <*> readTVar varReader
-    slotNoAtImmutableDBTip <- atomically $
-      ImmutableDB.getTipSlot cdbImmutableDB
-    findFirstPointOnChain curChain readerState slotNoAtImmutableDBTip pts
+    -- NOTE: we use `cdbChain` instead of 'Query.getCurrentChain', which only
+    -- returns the last @k@ headers, because we need to also see the headers
+    -- that happen to have not yet been GC'd over to the ImmutableDB.
+    join $ atomically $
+      findFirstPointOnChain
+        <$> readTVar cdbChain
+        <*> readTVar varReader
+        <*> ImmutableDB.getTipSlot cdbImmutableDB
+        <*> pure pts
   where
     findFirstPointOnChain ::
          HasCallStack