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>