Revert "src: close libuv handles on exit"

This change is not entirely ready for prime time: it's making ~50 tests
fail on Windows, mostly due to timeouts.  It's up for debate who is
at fault here: node.js or libuv.

It does however expose a libuv bug on OS X, where the event loop
sometimes gets stuck in uv__io_poll() when there is a single
UV_SHUTDOWN request left in the queue.  Needs further investigation.

This reverts commit 4915884da69814bd4daab22393919a628c9ecf23.
Revert "src: call uv_loop_delete() on exit in debug mode"

Don't call uv_loop_delete() until we've figured out a way to gracefully
close open handles.  See also commit 4915884 and its subsequent revert
in commit 980cbd5.

This reverts commit 556b890ad91780c52c77dccaddcf69beb8184e27.