[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/client.h

@@ -1302,6 +1302,7 @@ public:
 		return (task_state ? task_state->CompletedTasksInSet(task_set_id) : 0);
 	}
 	void PurgeTaskTimers();
+	void LockSharedTask(bool lock) { if (task_state) { task_state->LockSharedTask(this, lock); } }
 
 	// shared task shims / middleware
 	// these variables are used as a shim to intercept normal localized task functionality