BUG-8392 RpcRegistry has it's buckets populated by unreachable nodes

BUG-8392 RpcRegistry has it's buckets populated by unreachable nodes

In a situation when a member(f.ex member2) is isolated and the rpc registrations
are removed from the node(member1) we can still have our bucket store populated
by buckets from the remaining node(member-3) which might not have received
the memberUnreachable message yet leadig to stale routing of an rpc to
member-2.
This patch adds bucket filtering based on the currently present peers
so that we only accept Buckets that we can see.

Change-Id: I92c1e063f4754aca829bd73df4518f859e1d8497
Signed-off-by: Tomas Cere <tcere@cisco.com>