fs: fix nullCheck

* Call callback with FSReqWrap object for the function context.
* Make nullCheckSync variant
node: add CHECK to user facing MakeCallback API

In case a user calls node::MakeCallback() and the recv doesn't contain
a callback make sure to always abort.
doc: explain the req obj is now the callback ctx
WIP: async-wrap: use int64_t for uid

The maximum integer value in JS is 2^53. Meaning, if the uid was
increased every 100 ns there would be enough values to last over 28
years. That should be enough to suffice everyone.

Interestingly there's an issue that if async_id_ is changed to int64_t
it causes some strangeness to occur. Still figuring out why that is.