Use a proper mutex for autoloading features. (#5788)

Object#autoload implements a custom per-thread "mutex" for blocking
threads waiting on autoloading a feature. This causes problems when used
with the fiber scheduler. We swap the implementation to use a Ruby mutex
which is fiber aware.

test/fiber/autoload.rb

@@ -0,0 +1,3 @@
+sleep 0.01
+module TestFiberSchedulerAutoload
+end