Switch SerialExecutor to default to the immutable global executor

Summary: Use the global immutable executor by default for SerialExecutor.

Reviewed By: yfeldblum

Differential Revision: D26916419

fbshipit-source-id: c9b68c6b9facd11c8768199f8bae3d6c9276f4bd

folly/executors/SerialExecutor.h

@@ -58,8 +58,7 @@ class SerialExecutor : public SequencedExecutor {
   SerialExecutor& operator=(SerialExecutor&&) = delete;
 
   static KeepAlive<SerialExecutor> create(
-      KeepAlive<Executor> parent =
-          getKeepAliveToken(getUnsafeMutableGlobalCPUExecutor().get()));
+      KeepAlive<Executor> parent = getGlobalCPUExecutor());
 
   class Deleter {
    public: