If existing overviews we regenerate them
If existing overviews we regenerate them

Instead of make a DROP/CREATE always, in case we have existing
overviews we have to make DELETE/INSERT because DDL operations require
AccessExclusiveLock which is not compatible with AccessShareLock that
is used by the queries used to gather data for the tiler.

This incompatibility above mentioned leads to have deadlocks

CartoDB/cartodb-postgresql,CartoDB/cartodb-postgresql