[PATCH] group commit status variable addition

currently we have a status variable for the total group_commits. This compromises of group commits due to binlog_commit_wait_count, binlog_commit_wait_usec or indeed conflicting transactions that need to be in a separate group.

Showing a breakdown as to the reason of the group commit will enable a more informed tuning of the system variables controlling the group commit and also identify how often these aren't taking part in the group determination.

patch: https://github.com/MariaDB/server/pull/30