MDEV-19632 Replication aborts with ER_SLAVE_CONVERSION_FAILED upon CREATE ... SELECT in ORACLE mode

- Adding optional qualifiers to data types:
    CREATE TABLE t1 (a schema.DATE);
  Qualifiers now work only for three pre-defined schemas:

    mariadb_schema
    oracle_schema
    maxdb_schema

  These schemas are virtual (hard-coded) for now, but may turn into real
  databases on disk in the future.

- mariadb_schema.TYPE now always resolves to a true MariaDB data
  type TYPE without sql_mode specific translations.

- oracle_schema.DATE translates to MariaDB DATETIME.

- maxdb_schema.TIMESTAMP translates to MariaDB DATETIME.

- Fixing SHOW CREATE TABLE to use a qualifier for a data type TYPE
  if the current sql_mode translates TYPE to something else.

The above changes fix the reported problem, so this script:

    SET sql_mode=ORACLE;
    CREATE TABLE t2 AS SELECT mariadb_date_column FROM t1;

is now replicated as:

    SET sql_mode=ORACLE;
    CREATE TABLE t2 (mariadb_date_column mariadb_schema.DATE);

and the slave can unambiguously treat DATE as the true MariaDB DATE
without ORACLE specific translation to DATETIME.

Similar,

    SET sql_mode=MAXDB;
    CREATE TABLE t2 AS SELECT mariadb_timestamp_column FROM t1;

is now replicated as:

    SET sql_mode=MAXDB;
    CREATE TABLE t2 (mariadb_timestamp_column mariadb_schema.TIMESTAMP);

so the slave treats TIMESTAMP as the true MariaDB TIMESTAMP
without MAXDB specific translation to DATETIME.

sql/sql_schema.h

@@ -0,0 +1,70 @@
+#ifndef SQL_SCHEMA_H_INCLUDED
+#define SQL_SCHEMA_H_INCLUDED
+/*
+ Copyright (c) 2020, MariaDB Corporation.
+
+ This program is free software; you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation; version 2 of the License.
+
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with this program; if not, write to the Free Software
+ Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1335  USA */
+
+#include "mysqld.h"
+#include "lex_string.h"
+
+class Schema
+{
+  LEX_CSTRING m_name;
+public:
+  Schema(const LEX_CSTRING &name)
+   :m_name(name)
+  { }
+  virtual ~Schema() { }
+  const LEX_CSTRING &name() const { return m_name; }
+  virtual const Type_handler *map_data_type(THD *thd, const Type_handler *src)
+                                            const
+  {
+    return src;
+  }
+  /*
+    For now we have *hard-coded* compatibility schemas:
+      schema_mariadb, schema_oracle, schema_maxdb.
+    But eventually we'll turn then into real databases on disk.
+    So the code below compares names according to the filesystem
+    case sensitivity, like it is done for regular databases.
+
+    Note, this is different to information_schema, whose name
+    is always case insensitive. This is intentional!
+    The assymetry will be gone when we'll implement SQL standard
+    regular and delimited identifiers.
+  */
+  bool eq_name(const LEX_CSTRING &name) const
+  {
+#if MYSQL_VERSION_ID > 100500
+#error Remove the old code
+    return !table_alias_charset->strnncoll(m_name.str, m_name.length,
+                                           name.str, name.length);
+#else
+    // Please remove this when merging to 10.5
+    return !table_alias_charset->coll->strnncoll(table_alias_charset,
+                                                 (const uchar *) m_name.str,
+                                                 m_name.length,
+                                                 (const uchar *) name.str,
+                                                 name.length, FALSE);
+#endif
+  }
+  static Schema *find_by_name(const LEX_CSTRING &name);
+  static Schema *find_implied(THD *thd);
+};
+
+
+extern Schema mariadb_schema;
+
+#endif // SQL_SCHEMA_H_INCLUDED