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 "")

plugin/handler_socket/handlersocket/database.cpp

@@ -297,9 +297,9 @@ dbcontext::init_thread(const void *stack_bottom, volatile int& shutdown_flag)
       #else
       thd->options |= OPTION_BIN_LOG;
       #endif
-      safeFree(thd->db);
-      thd->db = 0;
-      thd->db = my_strdup("handlersocket", MYF(0));
+      safeFree((char*) thd->db.str);
+      thd->db.str= my_strdup("handlersocket", MYF(0));
+      thd->db.length= sizeof("handlersocket")-1;
     }
     thd->variables.option_bits |= OPTION_TABLE_LOCK;
     my_pthread_setspecific_ptr(THR_THD, thd);
@@ -1005,8 +1005,9 @@ dbcontext::cmd_open(dbcallback_i& cb, const cmd_open_args& arg)
     bool refresh = true;
     const thr_lock_type lock_type = for_write_flag ? TL_WRITE : TL_READ;
     #if MYSQL_VERSION_ID >= 50505
-    tables.init_one_table(arg.dbn, strlen(arg.dbn), arg.tbl, strlen(arg.tbl),
-      arg.tbl, lock_type);
+    LEX_CSTRING db_name=  { arg.dbn, strlen(arg.dbn) };
+    LEX_CSTRING tbl_name= { arg.tbl, strlen(arg.tbl) };
+    tables.init_one_table(&db_name, &tbl_name, 0, lock_type);
     tables.mdl_request.init(MDL_key::TABLE, arg.dbn, arg.tbl,
       for_write_flag ? MDL_SHARED_WRITE : MDL_SHARED_READ, MDL_TRANSACTION);
     Open_table_context ot_act(thd, 0);