Performance improvements for iterating over IDBCursor

Hello,

   In this pull request, I've added data pre-fetching when iterating over cursor. I.e. for continue() method,
   we take up to 100 records from SQL database and return results from this prefetched data.

   In my system, this gives speedup from 40 seconds to 4 second when loading 800 records from a large table with index (on iPad).

   Thanks,
   KIR