MDEV-4725: Incorrect binlog state recovery if crash while writing event group

The binlog state was not recovered correctly if XA is not used (eg. InnoDB
disabled), or if server crashed in the middle of writing an event group to the
binlog.

With this patch, we ensure that recovery of binlog state is done even if we do
not do the full XA binlog recovery, and we ensure that we only recover fully
written event groups into the binlog state.