Back out "Switch SerialExecutor to default to the immutable global executor"

Summary:
Original commit changeset: c9b68c6b9fac
Backout: D26916419 (https://github.com/facebook/folly/commit/b9f3d9be479d4dccdc8e5094933229e676308872)

Differential Revision: D28532140

fbshipit-source-id: 15e5eb1262234039387bfda36d280391f52cc3b2

folly/executors/SerialExecutor.h

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