- the mysql.ndb_schema table should not show up in "SHOW TABLES" or
   "select * from information_schema.tables"
 - fix by removing the now defunct find_files callback and instead mark
   the table as "hidden in SE" in DD. This also has the effect that
   neither DDL or DML queries on the table is possible
 - implement new test case checking behaviour of ndb_schema and remove
   previous test from ndb_multi
 - rewrite ndb_not_readonly.inc to not use mysql.ndb_schema to check
   if MySQL Server has com out of readonly mode.
 - remove writing of warning to error log when failure to acquire GSL
   lock due occurs. The mesage is still psuhed as warning.
 - remove "report_cluster_disconnected" parameter to
   Ndb_global_schema_lock_guard::lock() it's dead code where noone
   requested it to be on.