Merge from 10.0-base to 10.0 the feature MDEV-4506: Parallel replication.

The merge is still missing a few hunks related to temporary tables and
InnoDB log file size. The associated code did not seem to exist in
10.0, so the merge of that needs more work. Until this is fixed, there
are a number of test failures as a result.

sql/sql_parse.cc

@@ -4324,6 +4324,7 @@ end_with_restore_list:
     break;
 
   case SQLCOM_BEGIN:
+    DBUG_PRINT("info", ("Executing SQLCOM_BEGIN  thd: %p", thd));
     if (trans_begin(thd, lex->start_transaction_opt))
       goto error;
     my_ok(thd);