Preparatory changes for work shedding

- cleanup: change grpc_iomgr_cb_func to take a bool instead of int
  success
- cleanup: follow through with iomgr callback scheduling functions
- prepare: add a workqueue to offload to to grpc_exec_ctx_enqueue*
  functions
diff --git a/src/core/security/credentials.c b/src/core/security/credentials.c
index 8b56c57..afba007 100644
--- a/src/core/security/credentials.c
+++ b/src/core/security/credentials.c
@@ -793,7 +793,7 @@
 }
 
 static void on_simulated_token_fetch_done(grpc_exec_ctx *exec_ctx,
-                                          void *user_data, int success) {
+                                          void *user_data, bool success) {
   grpc_credentials_metadata_request *r =
       (grpc_credentials_metadata_request *)user_data;
   grpc_md_only_test_credentials *c = (grpc_md_only_test_credentials *)r->creds;
@@ -812,7 +812,7 @@
     grpc_credentials_metadata_request *cb_arg =
         grpc_credentials_metadata_request_create(creds, cb, user_data);
     grpc_executor_enqueue(
-        grpc_closure_create(on_simulated_token_fetch_done, cb_arg), 1);
+        grpc_closure_create(on_simulated_token_fetch_done, cb_arg), true);
   } else {
     cb(exec_ctx, user_data, c->md_store->entries, 1, GRPC_CREDENTIALS_OK);
   }