fs: forgot to call exists cb using "this"
async_listener: allow user to set cb to gen uid

Add setUidCb() so users can set a callback that will be called when a
new uid needs to be generated for a given AsyncWrap instantiation. The
value is still evaluated as int64_t.
src: cast uv_thread_self() to unsigned long

The update to libuv 1.0.0rc-2 introduced a compiler
error, as the return type of uv_thread_self() changed.
This commit restores the old behavior by casting the
return type.
test: fix test-tcp-wrap-listen.js