.all method _model gets redefined before callback

This was a fun one!

I was having problems with include getting mismatched model / include requests, but only some of the time.

I finally realized that these object / references, when defined outside the scope of the callback, can be altered via many sequential requests to `all` when rethink doesn't callback before to the next invocation of `all`.

More concretely, `all` gets called with model "foo" and include "bar". While that request is processing it gets called with model "baz". `_model` is now pointing at model "baz" when the "foo" request finishes, and attempts a bad include of "baz" with "bar".

This could be a problem in other places, I haven't yet been able to review the total library.