Fix SessionManager::Shutdown to actually shut-down sessions

SessionManager::Shutdown() wasn't actually shutting down any sessions
that were resident in the session table.

This meant that they would only get torn down on program termination,
which causes issues in the Python REPL since it results in Log prints
being emitted well after we've actually disconnected the logging
subsystem.

src/transport/SessionManager.cpp

@@ -111,6 +111,12 @@ void SessionManager::Shutdown()
         mFabricTable->RemoveFabricDelegate(this);
         mFabricTable = nullptr;
     }
+
+    mSecureSessions.ForEachSession([&](auto session) {
+        session->MarkForEviction();
+        return Loop::Continue;
+    });
+
     mMessageCounterManager = nullptr;
 
     mState        = State::kNotReady;