MDEV-26: Global transaction ID.

Fix yet another race in the rpl_gtid_startpos test case.

Implement include/wait_for_purge.inc to purge binary logs; we need to
retry the purge multiple times until it succeeds in removing all the logs
we want, as binlog dump threads can be slow to stop on loaded machines and
hold back purge of logs that are still referenced by the thread.

mysql-test/include/wait_for_purge.inc

@@ -0,0 +1,53 @@
+# include/wait_for_purge.inc
+#
+# SUMMARY
+#
+#    Repeatedly attempt to purge binlogs up to a specific point, until
+#    SHOW BINARY LOGS shows it has succeeded.
+#    This can be useful to avoid races where slaves are stopped but the
+#    corresponding binlog dump thread on the master has not yet terminated.
+#    Async binlog checkpoints can also delay purge.
+#
+#    Note that it may be necessary to inject at least one event in the binlog
+#    after stopping slaves, or master may not detect stop of slave for a long
+#    time (or forever), depending on details of test case, if replication
+#    heartbeat is enabled, etc.
+#
+# USAGE:
+#
+#    --let $purge_binlogs_to=master-bin.000003
+#    --source include/wait_for_purge.inc
+#
+# SIDE EFFECTS:
+#
+#    Disables --query_log while running, enables it afterwards.
+
+--echo include/wait_for_purge.inc "$purge_binlogs_to"
+
+let $_wait_count= 300;
+let $_done= 0;
+
+--disable_query_log
+while ($_wait_count)
+{
+  dec $_wait_count;
+  eval PURGE BINARY LOGS TO "$purge_binlogs_to";
+  let $_cur_binlog= query_get_value(SHOW BINARY LOGS, Log_name, 1);
+  if (`SELECT "$_cur_binlog" = "$purge_binlogs_to"`)
+  {
+    let $_done= 1;
+    let $_wait_count= 0;
+  }
+  if ($_wait_count)
+  {
+    real_sleep 0.1;
+  }
+}
+
+if (!$_done)
+{
+  SHOW FULL PROCESSLIST;
+  eval SHOW BINARY LOGS;
+  --die ERROR: failed while waiting for binlog purge to $purge_binlogs_to
+}
+--enable_query_log