The approach this patch is using is to do the commit stage update of the
GTIDs in a single "batch" without releasing the sidno_lock.

After the refactoring on the previous commit, this patch introduces the
update_commit_group function that works similarly to update_on_commit and
update_on_rollback functions, but doing the update on commit or rollback
in a set of transactions that were grouped to commit together.

This patch also introduced two new group commit versions of some functions
that were introduced on the refactoring:
- update_gtids_impl_lock_sidnos:
    Lock all the sidnos of all GTIDs to be updated in a given commit group;
- update_gtids_impl_broadcast_and_unlock_sidnos:
    Unlock all previously locked sidnos after broadcasting their changes;