Introduce grpc_completion_queue_factory API

Just the API and a bare-bone implementation
diff --git a/grpc.def b/grpc.def
index 30d60b0..1589316 100644
--- a/grpc.def
+++ b/grpc.def
@@ -53,6 +53,9 @@
     grpc_shutdown
     grpc_version_string
     grpc_g_stands_for
+    grpc_completion_queue_factory_lookup
+    grpc_completion_queue_create_for_next
+    grpc_completion_queue_create_for_pluck
     grpc_completion_queue_create
     grpc_completion_queue_next
     grpc_completion_queue_pluck