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.cc

@@ -0,0 +1,80 @@
+/*
+ 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 "mariadb.h"
+#include "sql_type.h"
+#include "sql_schema.h"
+#include "sql_class.h"
+
+class Schema_oracle: public Schema
+{
+public:
+  Schema_oracle(const LEX_CSTRING &name)
+   :Schema(name)
+  { }
+  const Type_handler *map_data_type(THD *thd, const Type_handler *src)
+                                    const
+  {
+    if (src == &type_handler_newdate)
+      return thd->type_handler_for_datetime();
+    return src;
+  }
+};
+
+
+class Schema_maxdb: public Schema
+{
+public:
+  Schema_maxdb(const LEX_CSTRING &name)
+   :Schema(name)
+  { }
+  const Type_handler *map_data_type(THD *thd, const Type_handler *src)
+                                    const
+  {
+    if (src == &type_handler_timestamp ||
+        src == &type_handler_timestamp2)
+      return thd->type_handler_for_datetime();
+    return src;
+  }
+};
+
+
+Schema        mariadb_schema(Lex_cstring(STRING_WITH_LEN("mariadb_schema")));
+Schema_oracle oracle_schema(Lex_cstring(STRING_WITH_LEN("oracle_schema")));
+Schema_maxdb  maxdb_schema(Lex_cstring(STRING_WITH_LEN("maxdb_schema")));
+
+
+Schema *Schema::find_by_name(const LEX_CSTRING &name)
+{
+  DBUG_ASSERT(name.str);
+  if (mariadb_schema.eq_name(name))
+    return &mariadb_schema;
+  if (oracle_schema.eq_name(name))
+    return &oracle_schema;
+  if (maxdb_schema.eq_name(name))
+    return &maxdb_schema;
+  return NULL;
+}
+
+
+Schema *Schema::find_implied(THD *thd)
+{
+  if (thd->variables.sql_mode & MODE_ORACLE)
+    return &oracle_schema;
+  if (thd->variables.sql_mode & MODE_MAXDB)
+    return &maxdb_schema;
+  return &mariadb_schema;
+}