net: add NetEventsInterface::g_msgproc_mutex

There are many cases where we assume message processing is
single-threaded in order for how we access node-related memory to be
safe. Add an explicit mutex that we can use to document this, which allows
the compiler to catch any cases where we try to access that memory from
other threads and break that assumption.

src/test/fuzz/process_messages.cpp

@@ -40,6 +40,8 @@ FUZZ_TARGET_INIT(process_messages, initialize_process_messages)
     SetMockTime(1610000000); // any time to successfully reset ibd
     chainstate.ResetIbd();
 
+    LOCK(NetEventsInterface::g_msgproc_mutex);
+
     std::vector<CNode*> peers;
     const auto num_peers_to_add = fuzzed_data_provider.ConsumeIntegralInRange(1, 3);
     for (int i = 0; i < num_peers_to_add; ++i) {