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/sql_partition.h

@@ -57,8 +57,8 @@ typedef struct st_lock_param_type
   Alter_info *alter_info;
   TABLE *table;
   KEY *key_info_buffer;
-  const char *db;
-  const char *table_name;
+  LEX_CSTRING db;
+  LEX_CSTRING table_name;
   uchar *pack_frm_data;
   uint key_count;
   uint db_options;
@@ -267,8 +267,8 @@ uint fast_alter_partition_table(THD *thd, TABLE *table,
                                 Alter_info *alter_info,
                                 HA_CREATE_INFO *create_info,
                                 TABLE_LIST *table_list,
-                                const char *db,
-                                const char *table_name);
+                                const LEX_CSTRING *db,
+                                const LEX_CSTRING *table_name);
 bool set_part_state(Alter_info *alter_info, partition_info *tab_part_info,
                     enum partition_state part_state);
 uint prep_alter_part_table(THD *thd, TABLE *table, Alter_info *alter_info,