src: add HandleScope in HandleWrap::OnClose

Fixes a 4 byte leak on handles closing. AKA The Walmart leak.

MakeCallback doesn't have a HandleScope. That means the callers scope
will retain ownership of created handles from MakeCallback and related.
There is by default a wrapping HandleScope before uv_run, if the caller
doesn't have a HandleScope on the stack the global will take ownership
which won't be reaped until the uv loop exits.

If a uv callback is fired, and there is no enclosing HandleScope in the
cb, you will appear to leak 4-bytes for every invocation. Take heed.

cc @hueniverse