Introduce grpc_completion_queue_factory API

Just the API and a bare-bone implementation
diff --git a/binding.gyp b/binding.gyp
index f79374a..957c343 100644
--- a/binding.gyp
+++ b/binding.gyp
@@ -711,6 +711,7 @@
         'src/core/lib/surface/channel_ping.c',
         'src/core/lib/surface/channel_stack_type.c',
         'src/core/lib/surface/completion_queue.c',
+        'src/core/lib/surface/completion_queue_factory.c',
         'src/core/lib/surface/event_string.c',
         'src/core/lib/surface/lame_client.c',
         'src/core/lib/surface/metadata_array.c',