The purpose of dict_table_t::stats_bg_flag was to prevent
race conditions between DDL operations and a background thread
that updates persistent statistics for InnoDB tables.

Now that with the parent commit, we started to acquire a
shared meta-data lock (MDL) on the InnoDB persistent statistics tables
in background tasks that access them, we may easily acquire MDL
on the table for which the statistics are being updated. This will by
design prevent race conditions with any DDL operations on that table,
and the stats_bg_flag may be removed.

dict_stats_process_entry_from_recalc_pool(): Complete rewrite.
During the processing, retain the entry in recalc_pool, so
that dict_stats_recalc_pool_del() will be able to request
deletion of the entry, or delete the entry if its caller is
holding MDL_EXCLUSIVE while we are waiting for MDL.

recalc_pool: In addition to the table ID, store a state for
inter-thread communication, so that dict_stats_recalc_pool_del()
can wait until all processing is finished.

Reviewed by: Thirunarayanan Balathandayuthapani