refactor def _get_dvr_sync_data from neutron/db/l3_dvr_db.py

The function def _get_dvr_sync_data in neutron/db/l3_dvr_db.py is fetching and processing routers data and since its called upon for each dvr ha router type on update, its becomes very hard to pin point issues in such a massive method, so I propose breaking it into two methods.

def _get_dvr_sync_data and _process_dvr_sync_data. will make debugging in future easy.