A new Server_state_observer hook "after_dd_upgrade_from_57" is introduced in
sql/replication.h. Plugins can use this hook to register a function
which will be run after the migration of mysql 5.7 metadata into the
Data Dictionary.

wl#12413 removed the mysql_upgrade utility script and replaced it with a
self-contained procedure inside the server. The new upgrade procedure runs
earlier than the old one did, possibly before NDB is fully ready to handle
user queries. This caused the code for migrating MySQL's privilege tables
from NDB storage to standard InnoDB storage in 8.0 to fail (see wl#12507
and wl#12711).

In this patch, the NDB plugin uses the new after_dd_upgrade_from_57 hook
to register a function that can handle the table migration at the proper
time. The patch also adds some logic into the DD upgrade code to correctly
create DD metadata for NDB distributed privilege tables, and it deletes the
SQL script ndb_upgrade_privileges.sql which had been added in wl#12507.

Change-Id: I94401b5184dd0291aca38d10cb6f0c12f27ec1c7