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_utils.h

@@ -28,7 +28,7 @@ public:
     if (error)
       return;
 
-    table.mdl_request.init(MDL_key::TABLE, table.db, table.table_name, MDL_EXCLUSIVE, MDL_EXPLICIT);
+    table.mdl_request.init(MDL_key::TABLE, table.db.str, table.table_name.str, MDL_EXCLUSIVE, MDL_EXPLICIT);
     error= thd->mdl_context.acquire_lock(&table.mdl_request, thd->variables.lock_wait_timeout);
     thd->mdl_context.release_lock(protection_request.ticket);
   }