Limit pool garbage collection to once per epoch.

Before this change, it was possible for garbage collection to happen
more than once per epoch, as any rollback would recreate the mutable
reference to the latest garbage collection epoch, causing the latest
garbage collection epoch value to be reset to zero.

This change adjusts the reference creation mechanism so that it only
happens once, on startup.

lib/shelley/src/Cardano/Wallet/Shelley/Pools.hs

@@ -463,19 +463,26 @@ monitorStakePools
     -> NetworkLayer IO t (CardanoBlock sc)
     -> DBLayer IO
     -> IO ()
-monitorStakePools tr gp nl db@DBLayer{..} = do
-    latestGarbageCollectionEpochRef <- mkLatestGarbageCollectionEpochRef
-    let forwardHandler = forward latestGarbageCollectionEpochRef
-    cursor <- initCursor
-    traceWith tr $ MsgStartMonitoring cursor
-    follow nl (contramap MsgFollow tr) cursor forwardHandler getHeader >>= \case
-        FollowInterrupted -> traceWith tr MsgHaltMonitoring
-        FollowFailure -> traceWith tr MsgCrashMonitoring
-        FollowRollback point -> do
-            traceWith tr $ MsgRollingBackTo point
-            liftIO . atomically $ rollbackTo point
-            monitorStakePools tr gp nl db
+monitorStakePools tr gp nl DBLayer{..} =
+    monitor =<< mkLatestGarbageCollectionEpochRef
   where
+    monitor latestGarbageCollectionEpochRef = loop
+      where
+        loop = do
+            cursor <- initCursor
+            traceWith tr $ MsgStartMonitoring cursor
+            let followTrace = contramap MsgFollow tr
+            let forwardHandler = forward latestGarbageCollectionEpochRef
+            follow nl followTrace cursor forwardHandler getHeader >>= \case
+                FollowInterrupted ->
+                    traceWith tr MsgHaltMonitoring
+                FollowFailure ->
+                    traceWith tr MsgCrashMonitoring
+                FollowRollback point -> do
+                    traceWith tr $ MsgRollingBackTo point
+                    liftIO . atomically $ rollbackTo point
+                    loop
+
     GenesisParameters
         { getGenesisBlockHash
         , getEpochStability