fuzz: share a single mocked steady clock across FuzzedSock instances

Each FuzzedSock used to own its mocked steady clock and call
MockableSteadyClock::SetMockTime() directly. Hold the clock by reference
to an externally provided FakeSteadyClock instead, so that several
FuzzedSock instances sharing a test case (e.g. one per peer, or one
created via Accept()) advance a single mocked clock, and the mocking goes
through the FakeSteadyClock RAII helper that resets mocktime on
destruction.

FakeSteadyClock is a LimitOne type, so each fuzz target constructs one
instance per iteration and passes it to ConsumeSock / ConsumeNode / the
FuzzedSock constructor.

src/test/fuzz/process_message.cpp

@@ -84,6 +84,7 @@ FUZZ_TARGET(process_message, .init = initialize_process_message)
     auto& chainman{static_cast<TestChainstateManager&>(*node.chainman)};
     const auto block_index_size{WITH_LOCK(chainman.GetMutex(), return chainman.BlockIndex().size())};
     FakeNodeClock clock{1610000000s}; // any time to successfully reset ibd
+    FakeSteadyClock steady_clock;
     chainman.ResetIbd();
     chainman.DisableNextWrite();
 
@@ -111,7 +112,7 @@ FUZZ_TARGET(process_message, .init = initialize_process_message)
 
     node.validation_signals->RegisterValidationInterface(node.peerman.get());
 
-    CNode& p2p_node = *ConsumeNodeAsUniquePtr(fuzzed_data_provider).release();
+    CNode& p2p_node = *ConsumeNodeAsUniquePtr(fuzzed_data_provider, steady_clock).release();
 
     connman.AddTestNode(p2p_node);
     FillNode(fuzzed_data_provider, connman, p2p_node);