Relation membership for child relations should be cached before hook execution
When executed, relation hooks compute the membership of the implied
relation as part of the watch used by the unit relation
lifecycle. However, other membership for other relations is computed
upon the first use of the relation-list command, then cached.
This should be changed such that membership is computed for all
relations. Presumably this can be done such that it is point-in-time
consistent by the use of multi-operation support in the ZK client.