[Fix #814] Use `cider-current-ns` instead of `nrepl-buffer-ns`

Other changes:

   - Don't reset `nrepl-buffer-ns` in default response handler.

   - Remove `cider-find-ns` (it's confusing to have two functions doing almost
     the same thing).

   - Use nil global value of `nrepl-buffer-ns`. This will allow easy detection
     of the cases when this variable shouldn't be used.