Pagination support in dbcore

The creation of this issue has been discussed in #736.
To implement pagination properly in the HTTP API, we need to add support for pagination in dbCore. Ideally the pagination mechanism should also be usable with the command-line interface.

Ideas about pagination have been discussed in #718. It seems were are basically 4 options :
- the naive approach : using start/offset parameters in query. While this is trivial to implement it is definitely not very efficient.
- use a seek-able id in the query ( i.e. "gives me the next items starting with this one") : this is the approach [recommended by sqlite](http://www.sqlite.org/cvstrac/wiki?p=ScrollingCursor), we have to make a small prototype to make sure it's doable with complex queries.
- when performing a query, store temporarily the result and return a token to the client, which can be used to walk through the ResultSet. The difficult part here is to maintain, and discard at some point, the ResultSets
- last solution : only return id for complex queries. This looks nice but I'm afraid it might not play well with the current query mechanism, which fetches full rows anyway.