refactor: Use MockableSteadyClock in ReportHeadersPresync

This allows the clock to be mockable in tests. Also, replace cs_main
with GetMutex() while touching this function.

Also, use the ElapseSteady test helper in the p2p_headers_presync fuzz
target to make it more deterministic.

The m_last_presync_update variable is a global that is not reset in
ResetAndInitialize. However, it is only used for logging, so completely
disable it for now.

Without this patch, the tool would report a diff:

cargo run --manifest-path ./contrib/devtools/deterministic-fuzz-coverage/Cargo.toml -- $PWD/bld-cmake/ $PWD/../qa-assets/fuzz_corpora/ p2p_headers_presync 32

...
  4468|     81|        auto now = std::chrono::steady_clock::now();
  4469|     81|        if (now < m_last_presync_update + std::chrono::milliseconds{250}) return;
-                                                                                        ^80
+                                                                                        ^79
...

src/test/fuzz/p2p_headers_presync.cpp

@@ -15,6 +15,7 @@
 #include <test/util/net.h>
 #include <test/util/script.h>
 #include <test/util/setup_common.h>
+#include <test/util/time.h>
 #include <uint256.h>
 #include <validation.h>
 
@@ -158,6 +159,9 @@ FUZZ_TARGET(p2p_headers_presync, .init = initialize)
 {
     SeedRandomStateForTest(SeedRand::ZEROS);
     FuzzedDataProvider fuzzed_data_provider{buffer.data(), buffer.size()};
+    // The steady clock is currently only used for logging, so a constant
+    // time-point seems acceptable for now.
+    ElapseSteady elapse_steady{};
     SetMockTime(ConsumeTime(fuzzed_data_provider));
 
     ChainstateManager& chainman = *g_testing_setup->m_node.chainman;