Fix EffectiveRibInWriter.writeTable()

Current implementation of table overwrite is not correct, as it does not actually wipe effective-rib-in table before creating it afresh.

This is slightly problematic, because it relies heavily on delta logic, forcing instantiations of DataObjectModifications. Furthermore it side-steps the need for RIBSupport to allow routes to be extract from before- and after-state, which is required for BGP-495.

Add support for adj-rib-in route extraction and fix writeTable() so that it wipes the table and re-creates all required routes.