Purge is slow due to repeatedly looking up missing or discarded table

The tests innodb.undo_truncate and innodb.full_crc32_import are occasionally failing due to not all transactions having been purged. This is deterministically repeatable on a slow VM when running an ASAN-instrumented executable.

The reason for this is that the purge worker threads are unnecessarily issuing repeated lookups for tables that have been dropped, rebuilt or discarded. The threads should remember the latest failed lookup, so that unnecessary contention on the InnoDB dict_sys->mutex and dict_operation_lock and the buffer pool pages for SYS_TABLES can be avoided.