Add concurrent execution helper to Worker

Concurrently executing a task on all workers *and* waiting until
all workers have executed the task seems to be common enough to
warrant a helper function for that purpose.

server/core/worker.cc

@@ -618,6 +618,7 @@ size_t Worker::execute_on_all(std::auto_ptr<DisposableTask> sTask)
 
     return n;
 }
+
 //static
 size_t Worker::execute_on_all_serially(Task* pTask)
 {
@@ -638,6 +639,13 @@ size_t Worker::execute_on_all_serially(Task* pTask)
     return n;
 }
 
+//static
+size_t Worker::execute_on_all_concurrently(Task* pTask)
+{
+    Semaphore sem;
+    return sem.wait_n(Worker::execute_on_all(pTask, &sem));
+}
+
 bool Worker::post_message(uint32_t msg_id, intptr_t arg1, intptr_t arg2)
 {
     // NOTE: No logging here, this function must be signal safe.