function Cursor (db, query, execFn) {
  this.db = db;
  this.query = query || {};
  if (execFn) { this.execFn = execFn; }
}
Create a new cursor for this collection
@param {Datastore} db - The datastore this cursor is bound to
@param {Query} query - The query this cursor will operate on
@param {Function} execFn - Handler to be executed after cursor has found the results and before the callback passed to find/findOne/update/remove