DatabaseTimerPersistence currently holds both portable and vendor-specific sql statements. When performing any database access, it tries to look up a vendor-specific sql statement, and then the portable one, so that the more optimized vendor-specific one has precedence.

Since a database type, once determined, is unlikely to change during the life time of a DatabaseTimerPersistence, we should be able to normalize all sql statements using the standard key, removing those sql statements (either portable ones or vendor-specific ones) from the cache. Any database access can directly look up by the standard key to get the most appropriate sql statement.