fs: remove makeCallback

There's no need to run fs callbacks in the global scope now that every
req creates its own object.
async_wrap, src: cleanup

Remove CATCH_RETURN_UNDEF and other minor things.
async_listener, cares_wrap: cleanup
env: simplify async_wrap calls

set_async_wrap_parent_class() is no longer a template method and
reset_async_wrap_parent_class() has been removed.
src: fix domain function casting

In debug mode V8 will fail on a bad cast. So only cast enter/exit if the
Value actually is a Function.