Add the ability to report known connected clients

Add the ability to report known connected clients

In order to allow from-scratch recovery, we need the ability to
query all existing shards to provide the identifiers of clients
they know about. This patch adds the models to support such a query
as well as implementing it in cluster-admin.

JIRA: CONTROLLER-1626
Change-Id: Id69aeb9021c8111dad10930620c9eaacfccd0d94
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit e32959e0bbc326f47c30ed7347f9a9af26813f89)
(cherry picked from commit e99739f7ee64c3281569a70d8795dea672979e3d)