This worklog is created for the backport of the WL#9837: "Group Replication:
Transaction savepoint support" to MySQL 5.7.19

This worklog goal is to improve Group Replication by adding support to
SAVEPOINT.

The SAVEPOINT statement sets a named transaction savepoint with a name of
identifier. If the current transaction has a savepoint with the same name, the
old savepoint is deleted and a new one is set.

The ROLLBACK TO identifier command reverts the state of the transaction back to
what was when executed the command SAVEPOINT identifier.

The RELEASE SAVEPOINT statement removes the named savepoint from the set of
savepoints of the current transaction. No commit or rollback occurs. It returns
an error if the savepoint does not exist.