The current semi-sync binlog fail-over recovery process uses
rpl_semi_sync_slave_enabled==TRUE as its condition to truncate a
primary server’s binlog, as it is anticipating the server to re-join
a replication topology as a replica. However, for servers configured
with both rpl_semi_sync_master_enabled=1 and
rpl_semi_sync_slave_enabled=1, if a primary is just re-started (i.e.
retaining its role as master), it can truncate its binlog to drop
transactions which its replica(s) has already received and executed.
If this happens, when the replica reconnects, its gtid_slave_pos can
be ahead of the recovered primary’s gtid_binlog_pos, resulting in an
error state where the replica’s state is ahead of the primary’s.

This patch changes the condition for semi-sync recovery to truncate
the binlog to instead use the configuration variable
--init-rpl-role, when set to SLAVE. This allows for both
rpl_semi_sync_master_enabled and rpl_semi_sync_slave_enabled to be
set for a primary that is restarted, and no transactions will be
lost, so long as --init-rpl-role is not set to SLAVE.

Reviewed By:
============
Sergei Golubchik <serg@mariadb.com>