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.