srvRelationUnitsWatcher defines the API wrapping a state.RelationUnitsWatcher.
It notifies about units entering and leaving the scope of a RelationUnit,
and changes to the settings of those units known to have entered.
Next returns when a change has occurred to an entity of the
collection being watched since the most recent call to Next
or the Watch call that created the srvRelationUnitsWatcher.
srvRelationUnitsWatcher defines the API wrapping a state.RelationUnitsWatcher. It notifies about units entering and leaving the scope of a RelationUnit, and changes to the settings of those units known to have entered.