Invalidate the backup_ids cache on backup_ids_temp table recreation

This was discovered as part of the research performed for MDL-33455.

Along the whole restore process we need to ensure that any destructive operation performed to the backup_ids_cache table (deletion of records, complete truncate, drop & recreate) leads to the invalidation of the in-memory backup_ids caches.

Luckily, the only destructive operation is that, in the middle of the process, once the restore prechecks have ended, the backup_ids_temp table may be dropped and recreated (optionally, depending of the results of the prechecks and the type of execution).

So we need to guarantee that, each time the backup_ids table is created, any existing cache referring to it is invalidated.

Results can be really disastrous if this isn't kept on sync. In fact they are (no user created at all, possible messing of parents information...).

So this is about to:

1) create one reset_backup_ids_cache() method that will invalidate the caches.
2) execute it each time the backup_ids_temp table is created.

Ciao