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_repl.cc

@@ -4265,7 +4265,7 @@ int log_loaded_block(IO_CACHE* file, uchar *Buffer, size_t Count)
     lf_info->last_pos_in_file= my_b_get_pos_in_file(file);
     if (lf_info->wrote_create_file)
     {
-      Append_block_log_event a(lf_info->thd, lf_info->thd->db, buffer,
+      Append_block_log_event a(lf_info->thd, lf_info->thd->db.str, buffer,
                                MY_MIN(block_len, max_event_size),
                                lf_info->log_delayed);
       if (mysql_bin_log.write(&a))
@@ -4273,7 +4273,7 @@ int log_loaded_block(IO_CACHE* file, uchar *Buffer, size_t Count)
     }
     else
     {
-      Begin_load_query_log_event b(lf_info->thd, lf_info->thd->db,
+      Begin_load_query_log_event b(lf_info->thd, lf_info->thd->db.str,
                                    buffer,
                                    MY_MIN(block_len, max_event_size),
                                    lf_info->log_delayed);