util: Add NodeClock::epoch alias

A default constructed time_point is the epoch, by definition.

Existing code uses a default constructed (or explicitly constructed with
a zero duration) chrono type to mean epoch. New code can now use
NodeClock::epoch as an alias.

src/util/time.cpp

@@ -27,6 +27,8 @@ static std::atomic<std::chrono::seconds> g_mock_time{}; //!< For testing
 std::atomic<bool> g_used_system_time{false};
 static std::atomic<MockableSteadyClock::mock_time_point::duration> g_mock_steady_time{}; //!< For testing
 
+static_assert(NodeClock::epoch.time_since_epoch().count() == 0);
+
 NodeClock::time_point NodeClock::now() noexcept
 {
     const auto mocktime{g_mock_time.load(std::memory_order_relaxed)};

src/util/time.h

@@ -23,6 +23,7 @@ struct NodeClock : public std::chrono::system_clock {
     static time_point now() noexcept;
     static std::time_t to_time_t(const time_point&) = delete; // unused
     static time_point from_time_t(std::time_t) = delete;      // unused
+    static constexpr time_point epoch{};
 };
 using NodeSeconds = std::chrono::time_point<NodeClock, std::chrono::seconds>;