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_cursor.cc

@@ -132,7 +132,7 @@ int mysql_open_cursor(THD *thd, select_result *result,
 
   MYSQL_QUERY_EXEC_START(thd->query(),
                          thd->thread_id,
-                         (char *) (thd->db ? thd->db : ""),
+                         thd->get_db(),
                          &thd->security_ctx->priv_user[0],
                          (char *) thd->security_ctx->host_or_ip,
                          2);
@@ -440,7 +440,7 @@ bool Select_materialize::send_result_set_metadata(List<Item> &list, uint flags)
   if (create_result_table(unit->thd, unit->get_column_types(true),
                           FALSE,
                           thd->variables.option_bits | TMP_TABLE_ALL_COLUMNS,
-                          "", FALSE, TRUE, TRUE, 0))
+                          &empty_clex_str, FALSE, TRUE, TRUE, 0))
     return TRUE;
 
   materialized_cursor= new (&table->mem_root)