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/lock.cc

@@ -1093,8 +1093,8 @@ void Global_read_lock::unlock_global_read_lock(THD *thd)
         int ret = wsrep->resync(wsrep);
         if (ret != WSREP_OK)
         {
-          WSREP_WARN("resync failed %d for FTWRL: db: %s, query: %s", ret,
-                     (thd->db ? thd->db : "(null)"), thd->query());
+          WSREP_WARN("resync failed %d for FTWRL: db: %s, query: %s",
+                     ret, thd->get_db(), thd->query());
           DBUG_VOID_RETURN;
         }
       }
@@ -1180,7 +1180,7 @@ bool Global_read_lock::make_global_read_lock_block_commit(THD *thd)
     if (rcode != WSREP_OK)
     {
       WSREP_WARN("FTWRL desync failed %d for schema: %s, query: %s",
-                 rcode, (thd->db ? thd->db : "(null)"), thd->query());
+                 rcode, thd->get_db(), thd->query());
       my_message(ER_LOCK_DEADLOCK, "wsrep desync failed for FTWRL", MYF(0));
       DBUG_RETURN(TRUE);
     }