Add WITH_LOCK macro: run code while locking a mutex

Results from ryanofksy suggestion on isPotentialTip/
waitForNotifications refactoring

src/sync.h

@@ -199,6 +199,16 @@ using DebugLock = UniqueLock<typename std::remove_reference<
         LeaveCritical();                                                       \
     }
 
+//! Run code while locking a mutex.
+//!
+//! Examples:
+//!
+//!   WITH_LOCK(cs, shared_val = shared_val + 1);
+//!
+//!   int val = WITH_LOCK(cs, return shared_val);
+//!
+#define WITH_LOCK(cs, code) [&] { LOCK(cs); code; }()
+
 class CSemaphore {
 private:
     std::condition_variable condition;