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/surface/channel_connectivity.c b/src/core/surface/channel_connectivity.c
index 10f5c4d..e2973bb 100644
--- a/src/core/surface/channel_connectivity.c
+++ b/src/core/surface/channel_connectivity.c
@@ -165,11 +165,11 @@
   }
 }
 
-static void watch_complete(grpc_exec_ctx *exec_ctx, void *pw, int success) {
+static void watch_complete(grpc_exec_ctx *exec_ctx, void *pw, bool success) {
   partly_done(exec_ctx, pw, 1);
 }
 
-static void timeout_complete(grpc_exec_ctx *exec_ctx, void *pw, int success) {
+static void timeout_complete(grpc_exec_ctx *exec_ctx, void *pw, bool success) {
   partly_done(exec_ctx, pw, 0);
 }