OPTION IS CHANGED

Problem and analysis:

After changing relay-log parameter content, a plug-in using the channel
service interface can make the server to fail when calling channel_create().

The channel service interface has introduced the change_master() function
parameter "preserve_log" in order to tell change_master() to not purge the
current relay logs for a channel regardless of the channel parameters to be
changed.

This is making change_master() function to reach a situation that is
impossible without using the channel service interface.

In the situation where a relay log info repository was "broken" because of
the change of relay-log parameter, doing a RESET SLAVE is enough to purge all
channel relay logs (that it assumes as missing because of the relay-log
change), and creating new ones with the new relay-log parameter.

As a plug-in may call the channel_create() function telling it to preserve
the relay logs, it would lead to a server failure because the server will
assume the relay log files to exist (and they may exist, but with another
name because of the relay-log parameter change).

Fix:

When the relay log repository is not initialized yet (not used or has failed
to initialize properly), the change_master function should not respect the
preserve_logs parameter used by channel_create() function.

Note: fixed also an issue in the channel service interface that was setting
the use of GTID auto positions even when asked to let it as default in a
change master operation.