common/wall_clock: Add virtual destructors

From -fsanitize=address, this code wasn't calling the proper destructor.
Adding virtual destructors for each inherited class and the base class
fixes this bug.

While we are at it, mark the functions as final.

src/common/wall_clock.h

@@ -13,6 +13,8 @@ namespace Common {
 
 class WallClock {
 public:
+    virtual ~WallClock() = default;
+
     /// Returns current wall time in nanoseconds
     [[nodiscard]] virtual std::chrono::nanoseconds GetTimeNS() = 0;