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/lex_string.h

@@ -0,0 +1,52 @@
+/*
+   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 LEX_STRING_INCLUDED
+#define LEX_STRING_INCLUDED
+
+typedef struct st_mysql_const_lex_string LEX_CSTRING;
+
+/* Functions to compare if two lex strings are equal */
+inline bool lex_string_cmp(CHARSET_INFO *charset,
+                           const LEX_CSTRING *a,
+                           const LEX_CSTRING *b)
+{
+  return my_strcasecmp(charset, a->str, b->str);
+}
+
+/*
+  Compare to LEX_CSTRING's and return 0 if equal
+*/
+inline bool cmp(const LEX_CSTRING *a, const LEX_CSTRING *b)
+{
+  return (a->length != b->length ||
+          memcmp(a->str, b->str, a->length));
+}
+
+/*
+  Compare if two LEX_CSTRING are equal. Assumption is that
+  character set is ASCII (like for plugin names)
+*/
+inline bool lex_string_eq(const LEX_CSTRING *a,
+                          const LEX_CSTRING *b)
+{
+  if (a->length != b->length)
+    return 1;                                   /* Different */
+  return strcasecmp(a->str, b->str) != 0;
+}
+
+#endif /* LEX_STRING_INCLUDED */