Avoid duplication of persistent callback handles in Database.each()

The current code for Database.each() creates persistent handles for its
callback functions in both "EachBaton" and "EachBaton->async" but only
uses the ones in "EachBaton->async".

So instead of duplicating the handles in "EachBaton->async" move the
ones from "EachBaton" there and replace them with dummies.
Merge branch 'master' of github.com:thierer/node-sqlite3