BUG 8318: Add a separate dispatcher for cluster actors

When the system is under load it seems like there can
be a missed heartbeat leading to false positives for unreachable
nodes. Run the actors responsible for heartbeats on a separate
dispatcher to avoid this.

Change-Id: Ib4f4225bf69e99d93e3c7010d6fbe1163b96a5a2
Signed-off-by: Tomas Cere <tcere@cisco.com>