This worklog implements a framework to do group-wide configuration
changes. After this worklog is done, the user will be able to change
single_primary_mode without having to stop group replication, and he
will also be able to trigger the election of a specific member as the
new primary.

In this worklog the following functions are introduced

 - group_replication_switch_to_multi_primary_mode()

This function makes the group switch from single primary mode to multi
primary mode.

 - group_replication_switch_to_single_primary_mode([server_uuid])

This function makes the group switch from multi primary mode to single
primary mode. If a valid server uuid is provided, the action will
elect that member as the new primary on the process.

 - group_replication_set_as_primary(server_uuid)

This function makes the group switch the primary from one member to
another without any member stops.