If a table has been dropped, rebuilt, or its tablespace has been
discarded or the table is corrupted, it does not make sense to
look up that table again while purging old undo log records.

purge_node_t::purge_node_t(): Replaces row_purge_node_create().

que_common_t::que_common_t(): Constructor.

row_import_update_index_root(): Remove the constant parameter
dict_locked=true, and update the table->def_trx_id in the cache.

purge_node_t::unavailable_table_id: The latest unavailable table ID,
to avoid future lookups.

purge_node_t::def_trx_id: The latest modification of the table
identified by unavailable_table_id, or TRX_ID_MAX.

purge_node_t::is_skipped(): Determine if a table should be skipped.

purge_node_t::skip(): Note that a table should be skipped.