Optimizations/improvements to _all_docs handler

* _all_docs should now be much faster for large databases since it
  doesn't have to read the body of each document (unless include_docs
  is enabled.) It still won't be blindingly fast, especially the first
  time it's called because it has to build a view index.
* The ?channels=true option is now available to regular users, not just
  admins. The channel list shows only channels available to the user.
Fixes #326