Retrofit updatable emulated scrollable cursors to have same behaviour as updatable server-side scrollable cursors

[mrotteveel]

The implementation of server-side scrollable cursors, when updatable, will add inserted rows at the end and retain deleted rows as an all-null row. The original implementation of emulated scrollable cursors will add an inserted row before the current row and remove the deleted row entirely.

The new behaviour should be retrofitted on the existing emulated scrollable cursor implementation, and make the database metadata report the right information. Using the FBUpdatableFetcher around FBCachedFetcher, and removing the manipulation of inserts/updated/deletes from FBCachedFetcher should be sufficient.

Scheduling for Jaybird 6 is tentative.