couch_query_server refactoring

[Benoit Chesneau]

I'm working on the couch_query_server refactoring:
    - extract it from the couch app
    - introduce generic way to add query servers written in Erlang or
      calling OS processes like couchjs (so rather than distinct os
processes from native only call an erlang module with some arguments)
   - split the couchapp engine in its own module.

I'm actually wondering why you have one proc / ddoc? Any reason for
that? Apart for the rewriter?

- benoît