node: Track last block that received a blockTip notification

Also signal m_tip_block_cv when StopRPC is called, for
consistency with g_best_block_cv. This is handled in
StopRPC instead of OnRPCStopped() because the latter
is deleted in a later commit.

Co-authored-by: TheCharlatan <seb.kung@gmail.com>
Co-authored-by: Ryan Ofsky <ryan@ofsky.org>

src/node/interfaces.cpp

@@ -139,7 +139,7 @@ public:
         // Stop RPC for clean shutdown if any of waitfor* commands is executed.
         if (args().GetBoolArg("-server", false)) {
             InterruptRPC();
-            StopRPC();
+            StopRPC(m_context);
         }
     }
     bool shutdownRequested() override { return ShutdownRequested(*Assert(m_context)); };