uv: revert change causing build failures

Revert uv_thread_self() to return unsigned long instead of uv_thread_t.
This was causing a build failure on Windows and is only a temporary fix
until the proper patch lands upstream.

Reverts: https://github.com/joyent/libuv/commit/59658a8
Fixes: ce112c2 "deps: update uv to v1.0.0-rc2"