common: Make use of [[nodiscard]] where applicable

Now that clang-format makes [[nodiscard]] attributes format sensibly, we
can apply them to several functions within the common library to allow
the compiler to complain about any misuses of the functions.

src/common/spin_lock.h

@@ -17,7 +17,7 @@ class SpinLock {
 public:
     void lock();
     void unlock();
-    bool try_lock();
+    [[nodiscard]] bool try_lock();
 
 private:
     std::atomic_flag lck = ATOMIC_FLAG_INIT;