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 "")

mysql-test/suite/perfschema/r/threads_mysql.result

@@ -11,7 +11,7 @@ name	thread/sql/main
 type	BACKGROUND
 processlist_user	NULL
 processlist_host	NULL
-processlist_db	NULL
+processlist_db	mysql
 processlist_command	NULL
 processlist_info	NULL
 unified_parent_thread_id	NULL
@@ -99,7 +99,7 @@ name	thread/sql/event_worker
 type	FOREGROUND
 processlist_user	root
 processlist_host	localhost
-processlist_db	NULL
+processlist_db	test
 processlist_command	Sleep
 processlist_info	SELECT SLEEP(3)
 unified_parent_thread_id	unified parent_thread_id