Properly reference count the WriteBufferBlobs in cursors

Cursors, being like read-only tables, need to do the same reference
counting for the WriteBufferBlobs as tables do. In particular they need
to take a reference on the blob store when the cursor is created (in
case the original table is modified or closed), and release the
reference when the cursor itself is closed.

src/Database/LSMTree/Internal.hs

@@ -881,12 +881,17 @@ newCursor !offsetKey th = withOpenTable th $ \thEnv -> do
     -- references to each run, so it is safe.
     allocTableContent reg contentVar = do
         RW.withReadAccess contentVar $ \content -> do
+          let wb      = tableWriteBuffer content
+              wbblobs = tableWriteBufferBlobs content
+          allocateTemp reg
+            (WBB.addReference wbblobs)
+            (\_ -> WBB.removeReference wbblobs)
           let runs = cachedRuns (tableCache content)
           V.forM_ runs $ \r -> do
             allocateTemp reg
               (Run.addReference r)
               (\_ -> Run.removeReference r)
-          pure (tableWriteBuffer content, runs)
+          pure (wb, runs)
 
 {-# SPECIALISE closeCursor :: Cursor IO h -> IO () #-}
 -- | See 'Database.LSMTree.Normal.closeCursor'.