src: fix MakeCallback implementations

Both node::MakeCallback() and AsyncWrap::MakeCallback should be passing
the process object as the context to {load,unload}AsyncQueue.

Also, the context of the caller should be passed as the first argument
to {load,unload}AsyncQueue. To accomplish this in node::MakeCallback the
"const" was removed.
async_wrap: fix bug that didn't allow after to run

Callbacks assigned to "after" didn't run because they weren't being
accessed properly from the asyncQueue object.