[Shared Tasks] Implement Activity Locking (#2339)

* Add shared task element locking

This adds the `lock_activity_id` field to the tasks table which will
automatically lock a shared task when that element becomes active.

A method was added to world analogous to zone's UnlockActivities to
determine when an activity is active with respect to task steps.

Also adds quest apis to manually lock or unlock a client's shared task

* Add comment

zone/task_client_state.h

@@ -56,6 +56,7 @@ public:
 	void CreateTaskDynamicZone(Client* client, int task_id, DynamicZone& dz);
 	void ListTaskTimers(Client* client);
 	void KickPlayersSharedTask(Client* client);
+	void LockSharedTask(Client* client, bool lock);
 
 	inline bool HasFreeTaskSlot() { return m_active_task.task_id == TASKSLOTEMPTY; }