Changed database, tablename and alias to be LEX_CSTRING

This was done in, among other things:
- thd->db and thd->db_length
- TABLE_LIST tablename, db, alias and schema_name
- Audit plugin database name
- lex->db
- All db and table names in Alter_table_ctx
- st_select_lex db

Other things:
- Changed a lot of functions to take const LEX_CSTRING* as argument
  for db, table_name and alias. See init_one_table() as an example.
- Changed some function arguments from LEX_CSTRING to const LEX_CSTRING
- Changed some lists from LEX_STRING to LEX_CSTRING
- threads_mysql.result changed because process list_db wasn't always
  correctly updated
- New append_identifier() function that takes LEX_CSTRING* as arguments
- Added new element tmp_buff to Alter_table_ctx to separate temp name
  handling from temporary space
- Ensure we store the length after my_casedn_str() of table/db names
- Removed not used version of rename_table_in_stat_tables()
- Changed Natural_join_column::table_name and db_name() to never return
  NULL (used for print)
- thd->get_db() now returns db as a printable string (thd->db.str or "")

sql/vers_string.h

@@ -1,3 +1,19 @@
+/*
+   Copyright (c) 2018, 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 02110-1301  USA */
+
 #ifndef VERS_STRING_INCLUDED
 #define VERS_STRING_INCLUDED
 
@@ -126,8 +142,7 @@ typedef XString<Compare_t, String> SString_t;
 
 
 #define XSTRING_WITH_LEN(X) (X).ptr(), (X).length()
-#define DB_WITH_LEN(X) (X).db, (X).db_length
-#define TABLE_NAME_WITH_LEN(X) (X).table_name, (X).table_name_length
-
+#define DB_WITH_LEN(X) (X).db.str, (X).db.length
+#define TABLE_NAME_WITH_LEN(X) (X).table_name.str, (X).table_name.length
 
 #endif // VERS_STRING_INCLUDED