BUG-6937: Add ReachableMember case to Gossiper

BUG-6937: Add ReachableMember case to Gossiper

In case we detect a member down event we remove that member's
address from the whitelist, leading to further gossips being
ignored.

Subscribe to ReachableMember event to receive notifications
when the cluster heals, so we propagate re-add the member
back to the whitelist.

Change-Id: Id6b366edfa2be89e1a15225d2cad786bbf552129
Signed-off-by: Tomas Cere <tcere@cisco.com>
Signed-off-by: Robert Varga <rovarga@cisco.com>
(cherry picked from commit b78ee4d6b08e2cc0cf5edd01af0e54c3bf619ab5)