MDEV-8134: The relay-log is not flushed after the slave-relay-log.999999 showed

Problem:
========
Auto purge of relaylogs stops when relay-log-file is
'slave-relay-log.999999' and slave_parallel_threads is enabled.

Analysis:
=========
The problem is that in Relay_log_info::inc_group_relay_log_pos() function,
when two log names are compared via strcmp() function, it gives correct
result, when log name sequence numbers are of same digits(6 digits), But
when the number goes to 7 digits, a 999999 compares greater than
1000000, which is wrong, hence the bug.

Fix:
====
Extract the numeric extension part of the file name, convert it into
unsigned long and compare.

Thanks to David Zhao for the contribution.

mysql-test/suite/rpl/t/rpl_relay_max_extension.test

@@ -0,0 +1,109 @@
+# ==== Purpose ====
+#
+# Test verifies that auto purging mechanism of relay logs works fine when the
+# file extension grows beyond 999999.
+#
+# ==== Implementation ====
+#
+# Steps:
+#    0 - In master-slave setup clear all the relay logs on the slave server.
+#    1 - Start the slave so that new relay logs starting from
+#        'slave-relay-bin.000001' are created.
+#    2 - Get the active relay-log file name by using SHOW SLAVE STATUS.
+#        Shutdown the slave server.
+#    3 - Rename active relay log to '999997' in both 'relay-log.info' and
+#        'slave-relay-bin.index' files.
+#    4 - Restart the slave server by configuring 'slave_parallel_threads=1'
+#        and 'max_relay_log_size=100K'.
+#    5 - Generate load on master such that few relay logs are generated on
+#        slave. The relay log sequence number will change to 7 digits.
+#    6 - Sync slave with master to ensure that relay logs are applied on
+#        slave. They should have been automatically purged.
+#    7 - Assert that there is no 'slave-relay-bin.999999' file in
+#        'relay-log.info'.
+#
+# ==== References ====
+#
+# MDEV-8134: The relay-log is not flushed after the slave-relay-log.999999
+#            showed
+#
+
+--source include/have_innodb.inc
+--source include/have_binlog_format_row.inc
+--let $rpl_topology=1->2
+--source include/rpl_init.inc
+
+--connection server_2
+--source include/stop_slave.inc
+RESET SLAVE;
+--source include/start_slave.inc
+--source include/stop_slave.inc
+--let $relay_log=query_get_value(SHOW SLAVE STATUS, Relay_Log_File, 1)
+
+--echo #
+--echo # Stop slave server
+--echo #
+
+--let $datadir = `select @@datadir`
+--exec echo "wait" > $MYSQLTEST_VARDIR/tmp/mysqld.2.expect
+--shutdown_server 10
+--source include/wait_until_disconnected.inc
+
+--exec sed -i "s/$relay_log/slave-relay-bin.999997/g" $datadir/relay-log.info
+--exec sed -i "s/$relay_log/slave-relay-bin.999997/g" $datadir/slave-relay-bin.index
+
+--echo #
+--echo # Simulate file number get close to 999997
+--echo # by renaming relay logs and modifying index/info files
+
+--move_file $datadir/$relay_log $datadir/slave-relay-bin.999997
+
+--echo #
+--echo # Restart slave server
+--echo #
+
+--exec echo "restart" > $MYSQLTEST_VARDIR/tmp/mysqld.2.expect
+--enable_reconnect
+--source include/wait_until_connected_again.inc
+SET @save_slave_parallel_threads= @@GLOBAL.slave_parallel_threads;
+SET @save_max_relay_log_size= @@GLOBAL.max_relay_log_size;
+
+SET GLOBAL slave_parallel_threads=1;
+SET GLOBAL max_relay_log_size=100 * 1024;
+--source include/start_slave.inc
+
+--connection server_1
+create table t1 (i int, c varchar(1024));
+--echo #
+--echo # Insert some data to generate enough amount of binary logs
+--echo #
+--let $count = 1000
+--disable_query_log
+while ($count)
+{
+  eval insert into t1 values (1001 - $count, repeat('a',1000));
+  dec $count;
+}
+--enable_query_log
+--save_master_pos
+
+--connection server_2
+--sync_with_master
+
+--let $relay_log=query_get_value(SHOW SLAVE STATUS, Relay_Log_File, 1)
+
+--echo #
+--echo # Assert that 'slave-relay-bin.999999' is purged.
+--echo #
+let SEARCH_FILE=$datadir/slave-relay-bin.index;
+let SEARCH_PATTERN=slave-relay-bin.999999;
+source include/search_pattern_in_file.inc;
+
+--source include/stop_slave.inc
+SET GLOBAL slave_parallel_threads= @save_slave_parallel_threads;
+SET GLOBAL max_relay_log_size= @save_max_relay_log_size;
+--source include/start_slave.inc
+
+--connection server_1
+DROP TABLE t1;
+--source include/rpl_end.inc