async_wrap: new instances get uid

New instances of AsyncWrap are automatically assigned a unique id. The
value will be used in future commits to communicate additional
information via the async hooks.

While the largest value we can reliably communicate to JS is 2^53, even
if a new AsyncWrap is created every 100ns the uid won't reach its end
for 28.5 years.

PR-URL: https://github.com/nodejs/node/pull/3461


Reviewed-By: Fedor Indutny <fedor@indutny.com>